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Microwave pultrusion of unidirectional reinforced epoxy composites using single-frequency and variable-frequency processing techniques

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M ICROW AVE PULTRUSION OF UNIDIRECTIONAL REINFORCED EPOXY
COM POSITES USING SINGLE FREQUENCY AND VARIABLE FREQUENCY
PROCESSING TECHNIQUES
By
Aaron C. Smith
A THESIS
Submitted to
Michigan State University
in partial fulfillment o f the requirements
for the degree o f
MASTER OF SCIENCE
Department o f Chemical Engineering
1999
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UMI Number: 1395451
UMI Microform 1395451
Copyright 1999, by UMI Company. All rights reserved.
This microform edition is protected against unauthorized
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ABSTRACT
MICROWAVE PULTRUSION OF UNIDIRECTIONAL REINFORCED EPOXY
COMPOSITES USING SINGLE FREQUENCY AND VARIABLE FREQUENCY
PROCESSING TECHNIQUES
By
Aaron C. Smith
The pultrusion process is a continuous manufacturing method, which can be used
to produce reinforced plastics with a constant cross-sectional area through the length o f
the product. The continuous microwave processing technique is highly desired for
processing large composite materials, which need even and fast heating such as long pipes
or panels. The general advantages o f using microwave technology include: decreased
processing times, better control o f temperature profiles within the composite material, and
enhanced mechanical properties.
The objective o f this project was to obtain experimental data for microwave
pultrusion processing. Heating and curing profiles o f the pultruded material were studied
at different processing conditions. The fixed frequency pultrusion tests were
accomplished using the best heating mode for the prepreg system at a certain processing
speed and input power. The relation between microwave processing rate, input power,
and extent o f cure was studied by finding the optimum operating conditions. Mechanical
tests, such as flexural and tensile strength measurements, were conducted. A variable
frequency power source was used to achieve a more even temperature distribution across
the prepreg. The effects o f fiber reinforcement on the efficiency o f microwave heating
were studied. Concepts for an on-line monitoring system were explored.
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To my wife, Jodi, for her unconditional love and encouragem ent throughout the duration
o f this project. H er understanding for irregular hours and late nights did not waver. I am
truly blessed to have her in my life.
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ACKNOWLEDGMENTS
The author acknowledges Sigma Technology and M ichigan State University for
their financial support. Appreciation is extended to Dr. M artin C. Hawley for his support
during this research project, for the intellectual discussions, and for a deeper
understanding o f chemical engineering.
The author is grateful for the CMSC and Chemical Engineering staffs. Their
kindness and hard w ork made graduate school less stressful. Thanks must be extended to
Mr. Michael Rich for his help with the CMSC instruments and for leading me to a great
career. Appreciation is given to Amy McMillan for preparing samples and charts and for
running analytical tests. The fellow colleagues are recognized for their assistance in the
laboratory: Mr. Phil Culcasi, Mr. Colin Durham, Mr. Bao Fu, Dr. Yong-Joon Lee, and
Mr. Yunchang Qiu. Thanks for the laughs and the deep office conversations.
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TABLE OF CONTENTS
LIST OF T A B L E S ....................................................................................................................... viii
LIST OF F IG U R E S .......................................................................................................................ix
KEY TO S Y M B O L S .................................................................................................................. xiii
IN T R O D U C T IO N ......................................................................................................................... 1
CH APTER 1
B A C K G R O U N D ............................................................................................................................ 2
1.1. Microwave P rocessing............................................................................................. 2
1.2. Microwave Heating M echanism ............................................................................ 3
1.3. Microwave Heating A dvantages............................................................................ 5
1.4. Microwave A pplicators...........................................................................................6
1.5. Research Scope and G o a ls ...................................................................................... 8
CH APTER 2
APPLICATION OF CONTINUOUS MICROWAVE PR O C ESSIN G ......................... 10
2.1. Introduction........................................................................................................ 10
2.2. P ultrusion........................................................................................................... 11
2.3. Experimental S e tu p .............................................................................................14
2.3.1. C avity................................................................................................. 15
2.3.2. Calibration o f R o llers.................................................................... 17
2.4. Materials: Preparation and D iscussion......................................................... 17
2.5. G o a ls ................................................................................................................... 19
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CHAPTER 3
CAVITY CH A RA CTERIZATION......................................................................................20
3.1. Finding Electromagnetic M odes......................................................................20
3.2. Temperature M easurem ent.............................................................................. 22
3.3. Heating Studies At Various M o d es................................................................ 23
3.4. Preliminary Heating Studies in T E 112 M o d e ................................................. 26
3.5. C onclusions..............................................................................................................27
CHAPTER 4
FIXED FREQUENCY PULTRUSION EX PERIM EN TS................................................... 29
4.1. B ack g ro u n d .............................................................................................................29
4.2. Experimental S e tu p ............................................................................................... 29
4.3. Variation in Pulling S p eed ................................................................................ 30
4.4. Variation in Input P o w e r.................................................................................. 31
4.5. Extent o f Cure M easurem ent........................................................................... 31
4.6. Mechanical Properties M easurem ent.............................................................. 38
4.6.1. Tensile S trength ................................................................................. 38
4.6.2. Flexural S trength............................................................................... 41
4.7. C onclusions......................................................................................................... 45
CHAPTER 5
VARIABLE FREQUENCY PULTRUSION EXPERIMENTS ...................................... 46
5.1. Background On Variable Frequency P rocessing............................................ 46
5.2. Frequency Sweep Analysis................................................................................48
5.3. Input Power vs. Frequency............................................................................... 50
5.4. Temperature Measurement and Heating Studies Part I: 6-ply Graphite/Epoxy
Prepreg, Four Frequencies For V.F. Processing..................................................... 51
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5.5. Heating Studies Part II: 18-ply Graphite/Epoxy Prepreg, Change In Cavity
D im ension....................................................................................................................... 52
5.6. Heating Studies Part HI: 11-ply Graphite/Epoxy Prepreg, Four Frequencies
and Time In terv als...........................................................................................................56
5.7.
Heating Studies Part IV: 11-ply Graphite/Epoxy Prepreg, Four
Frequencies and Variation In The Time In te rv a l.........................................................60
5.8. Extent o f Cure M easurement.................................................................................62
5.9. C onclusions..............................................................................................................63
CHAPTER 6
FIBER REINFORCEM ENT EFFECTS ON M ICROW AVE PROCESSING
E F FIC IE N C Y ................................................................................................................................ 64
6.1. Material Identification and Purpose o f S tu d y ..................................................... 64
6.2. E xperim ents.............................................................................................................. 65
6.3. Results and D iscussion.............................................................................................67
6.4. C onclusions............................................................................................................... 70
CHAPTER 7
ON-LINE M O N IT O R IN G ..........................................................................................................71
7.1. Possible On-line Monitoring T echniques............................................................ 71
7.2. Pultrusion Model for On-Line M onitoring ..........................................................73
7.3. Conclusions on On-Line M onitoring..................................................................79
C O N C LU SIO N S.......................................................................................................................... 80
RECO M M EN D A TIO N S.......................................................................................................... 82
BIBLIO G R A PH Y ....................................................................................................................... 86
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LIST OF TABLES
Table 1. Calibration o f Rollers To Evaluate Possible Processing S p eed s........................ 17
Table 2. Heating M odes for the Cavity with Teflon D ie ...................................................... 24
Table 3. Heating M odes for the Cavity with Teflon Die and
Graphite/Epoxy Prepreg .. 24
Table 4. Heating M odes for the Cavity with Teflon Die and
Glass/Epoxy P re p re g ....24
Table 5. Extent o f Cure Measurement For Preliminary Heating o f Six-Ply Graphite/Epoxy
Composite in T E u 2 M ode With Input Power V a rie d .............................................................. 27
Table 6.
Extent o f Cure o f 11-ply Prepreg With Variable Frequency P rocessing
Table 7. Heating M odes for the Cavity with Teflon Die and Glass/Epoxy P re p re g
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65
LIST OF FIGURES
Figure 1. M icrowave Pultrusion Experimental Apparatus at M S U ................................. 15
Figure 2. Schematic o f Microwave Applicator at M S U ...................................................... 16
Figure 3. Entrance To Microwave Cavity Through Teflon D i e ........................................ 16
Figure 4. Frequency vs. Cavity Length for 7.00" +/- 0.001" (TE m o d es)...................... 21
Figure 5. Frequency vs. Cavity Length of 7.00" +/- 0.001" (TM m o d es)....................... 22
Figure 6. Teflon Die Used for Heating Studies and M icrowave Pultrusion Processing .. 23
Figure 7. Temperature Versus Time Results for Heating Studies o f Six-Ply Hercules
AS4/3501-6 Prepreg at 100 Watts and 0.787 ft/m in ...............................................................25
Figure 8. Temperature Versus Time Results for Heating Studies o f Stationary Six-Ply
DA409/E250 Prepreg At Approximately 50 W a tts ................................................................25
Figure 9. Results For Preliminary Heating of Six-ply Graphite/Epoxy Composite In T E m
Mode At Various Input P o w e rs..........................................................................................26
Figure 10. Effect o f Input Power on Extent o f C u re ..................................................... 34
Figure 11. Trendline for Extent o f Cure as Function o f Input Pow er (40-70 Watts) ... 34
Figure 12. Trendline for Extent o f Cure as Function o f Input Pow er (70-100 Watts)...35
Figure 13. Effect o f Pulling Speed on Extent o f C u r e .......................................................... 36
Figure 14. Trendline For Extent O f Cure As Function O f Pulling Speed (0.281-0.562
ft/m in )............................................................................................................................................ 36
Figure 15. Trendline For Extent O f Cure As Function O f Pulling Speed (0.562-0.733
ft/m in )..............................................................................................................................................37
Figure 16. Trendline For Extent O f Cure As Function O f Pulling Speed (0.733-1.17
ft/m in )..............................................................................................................................................37
Figure 17. Tensile Strength o f Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 ft/min and Variable Input P o w e r.................................................................... 39
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Figure 18. Tensile Modulus of Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 ft/min and Variable Input P o w e r....................................................................39
Figure 19. Tensile Strength o f Six-ply Graphite/Epoxy Prepreg Tape with An Input
Power o f 100 Watts and Variable Pulling S p eed..................................................................40
Figure 20. Tensile Modulus o f Six-ply Graphite/Epoxy Prepreg Tape with An Input
Power o f 100 Watts and Variable Pulling S p eed ................................................................... 40
Figure 21. Flexural Strength of Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 fl/min and Variable Input P o w e r................................................................... 42
Figure 22. Flexural Modulus o f Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 ft/min and Variable Input P o w e r................................................................... 42
Figure 23. Flexural Strength o f Six-ply Graphite/Epoxy Prepreg Tape with An Input
Power o f 100 Watts and Variable Pulling S p eed................................................................... 43
Figure 24. Flexural Modulus of Six-ply Graphite/Epoxy Prepreg Tape with An Input
Power o f 100 Watts and Variable Pulling S peed ................................................................... 43
Figure 25. Six Ply Hercules AS4/3501-6 Heated in T E n 2 mode at 100 Watts and 0.281
ft/m in ........................................................................................................................ 47
Figure 26. Six Ply Hercules AS4/3501-6 Heated in TEou mode at 100 Watts and 0.281
ft/m in ........................................................................................................................ 48
Figure 27. Percentage o f Reflected Pow er At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 90 mm And dp = 15.89 mm And Containing Sixply Hercules AS4/3501-6 P rep reg ............................................................................................ 49
Figure 28. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 135 mm And dp = 15.89 mm And Containing
Six-ply Hercules AS4/3501-6 P rep reg .....................................................................................50
Figure 29. Incident Pow er Versus Processing Frequency for Lambda Variable Frequency
Power Source (R E F ).................................................................................................................. 51
Figure 30. Six-Ply Hercules AS4/3501-6 Heated With Variable Frequency Processing
(2.4285 GHz, 2.444 GHz, 2.594 GHz, and 2.6140 GHz - 100 ms each) At An Average O f
70 Watts And A Pulling Speed of 0.281 ft/m in...................................................................... 52
Figure 31. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 90 mm And dp = 15.89 mm And Containing
Eighteen-ply Hercules AS4/3501-6 P re p re g .......................................................................... 53
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Figure 32. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 90 mm And dp = 22.90 mm And Containing
Eighteen-ply Hercules AS4/3501-6 P rep reg .............................................................................54
Figure 33. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 93 mm And dp = 22.90 mm And Containing
Eighteen-ply Hercules AS4/3501-6 P rep reg .............................................................................55
Figure 34. Eighteen-Ply Hercules AS4/3501-6 H eated With Variable Frequency
Processing (2.1173 GHz, 2.4162 GHz, and 2.7014 GHz - 1000ms each) and A Pulling
Speed o f 0.281 ft/m in ....................................................................................................................56
Figure 35. Eleven Ply Hercules AS4/3501-6 H eated With Variable Frequency Processing
(2.1173 GHz - 1000ms, 2.4162 GHz - 10ms, 2.6328 GHz - 1000 ms, 2.6809 GHz 1000ms) and Using A Pulling Speed O f 0.281 ft/m in .............................................................57
Figure 36. Eleven Ply Hercules AS4/3501-6 H eated With Variable Frequency Processing
(2.4162 GHz - 1000 ms, 2.4281 GHz - 1000 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed O f 0.281 ft/m in ............................................................58
Figure 37. Eleven Ply Hercules AS4/3501-6 H eated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4281 GHz - 2000 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed O f 0.281 ft/m in ............................................................59
Figure 38. Eleven Ply Hercules AS4/3501-6 H eated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4287 GHz - 2000 ms, 2.6328 GHz - 7000 ms, 2.6809 GHz 7000 ms) and Using A Pulling Speed o f 0.281 ft/m in ............................................................. 59
Figure 39. Eleven Ply Hercules AS4/3501-6 H eated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4287 GHz - 1000 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed o f 0.281 ft/m in .............................................................61
Figure 40. Eleven Ply Hercules AS4/3501-6 H eated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4287 GHz - 1500 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed of 0.281 ft/m in .............................................................61
Figure 41. Temperature Versus Time Results for Heating Studies o f Stationary Six-Ply
DA490/250 Prepreg At Approximately 50 W a tts ....................................................................66
Figure 42. Stationary Six-ply Glass/Epoxy Prepreg Heated in T E u 2 Mode At 57 Watts
for Two M in u tes............................................................................................................................67
Figure 43. Centerline Temperature for Six-ply Graphite/Epoxy Prepreg Heated in T E n2
Mode A t 50 W atts and 0.281 ft/m in ..........................................................................................68
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Figure 44. Six-ply Glass/Epoxy Prepreg Heat in T E 112 Mode At 50 W atts and 0.281
ft/m in ................................................................................................................................................ 68
Figure 45. Stationary Six-ply Glass/Epoxy Prepreg Heated in T E 112 M ode At 85 Watts
for Five M inutes............................................................................................................................. 69
Figure 46. Centerline Temperature for Six-ply Graphite/Epoxy Prepreg Heated in TEi 12
Mode At 80 Watts and 0.281 ft/m in ....................................................................................... 70
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KEY TO SYMBOLS
a [=] extent o f cure
P [=] ratio o f initial hardener equivalents to epoxide equivalents
5 [=] penetration depth
d [=] partial derivative
eo [=] permitivity o f free space (8.854 x 10'14 F/m)
e ! [=] dielectric constant
s* [=] effective complex permitivities
s ” [=] effective dielectric loss factor
sa” [=] loss factor due to the dielectric
/ [=] frequency (Hz, MHz, or GHz)
X [=] wavelength (cm)
Po [=] permeability o f free space
M-’ [=] permeability o f material
<d [=] 2*7t*/ (H z)
7t [=] 3.14159
p [=] density (g/cm5)
a [=] electrical conductivity (S/m)
CTfi, [=] ultimate strength o f fiber (MPa)
ctib [=]
ultimate strength o f composite (MPa)
CTmfd [=] matrix stress at the onset o f fiber cracking (MPa)
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tan5e [=] dielectric loss tangent
0 [=] angular direction in cylindrical coordinate system
c [=] speed o f light (3.00 x 108 m/s)
°C [=] Celsius, a unit o f tem perature
cm [=] centimeter, a unit o f length
cos [=] trigonometric cosine function
Cpf [=] heat capacity o f fiber (J/g_K)
Cpm [=] heat capacity o f matrix (J/g_K)
D [=] diameter o f brass cavity (cm or in)
dp [=] coupling probe depth (mm)
DSC [=] Differential Scanning Calorimetry
E [=] electric field vector quantity
1Em | [=] magnitude o f electric field inside the material
£ [=] 2.45 x 109 Hz
ft [=] feet, a unit o f length
g [=] gram, a unit o f weight
GHz [=] Gigahertz (109 Hz)
H [=] magnetic field vector quantity
I Hra I [=] magnitude o f magnetic field
H [=] rate o f heat generation per unit weight by the polymerization reaction (W/g)
Hr [=] heat o f reaction
H rc [=] heat o f reaction for cured material (J/g)
H m [=] heat o f reaction for uncured material (J/g)
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Hz [=] hertz, a unit o f frequency
in [=] inch, a unit o f length
j [=] imaginary number
J [=] joule, a unit o f energy
Ji(x) [=] Bessel Function
J’i(x) [=] Henckel Function
k [=] thermal conductivity (W/cm_K), o r kinetic rate constant
°K [=] Kelvin, a unit o f temperature
1 [=] number o f full-period variations o f Er with respect to 0
Ibf [=] pound force
Lc [=] cavity length (cm)
Lp [=] coupling probe depth (mm)
m [=] meter, a unit o f length, or
number o f half-period variations o f E0 with respect to r
MHz [=] megahertz (106 Hz)
min [=] minute, a unit o f time
mm [=] millimeter, a unit o f length
MPa [=] megapascal, a unit o f pressure or force
ms [=] millisecond, a unit o f time
N [=] number o f half-period variations o f Er with respect to z
P [=] microwave input power (W)
Pabs [=] power absorption per unit volume (W/cm3)
Pm’ [=] rate o f heat generation per unit volume by absorbed microwave energy (W/cm'’)
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r [=] radius, or radial direction
R [=] universal gas constant (8.314 J/moI_K)
R.P. [=] reflected power (W)
s [=] second, a unit o f time
sec [=] second, a unit o f time
sin [=] trigonometric sine function
t [=] time
T [=] temperature
Ta [=] ambient temperature
Tc [=] curing temperature
TE [=] transverse electric
TM [=] transverse magnetic
T.W.T. [=] traveling wave tube
Vz [=] pulling speed
W [=] watts, a unit o f power, or energy per time
Wf [=] weight fraction o f fiber in composite
wm [=] weight fraction o f matrix in composite
x [=] plane across width o f prepreg
xim [=] mth root o f Henckel Function for the TE-modes or
mth root o f Bessel Function for TM-modes
z [=] axial distance or direction in brass cavity, or plane o f pulling direction
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INTRODUCTION
The pultrusion process is a continuous manufacturing method which can be used
to produce reinforced plastics with a constant cross-sectional area through the length o f
the product. Pultruded samples are expected to exhibit good hydrolytic stability,
improved mechanical strength, and electrical insulation properties. The continuous
microwave processing technique is highly desired for processing large composite
materials, which need even and fast heating such as long pipes or panels. The general
advantages o f using microwave technology include: decreased processing times, better
control o f temperature profiles within the composite material, and enhanced mechanical
properties.
The objectives o f this project are to obtain experimental data o f microwave
pultrusion processing and to compare variable frequency and single frequency processing
methods. Temperature-time curves and extent o f cure measurements will aid in finding
the optimized microwave pultrusion variables o f input power and pulling speed.
Mechanical tests, such as flexural and tensile strength measurements, will be conducted.
A variable frequency power source will be used to determine if mode-switching heating
can provide more even heating in pultruded composites than with fixed frequency heating.
By processing continuous, unidirectional glass and graphite reinforced epoxy prepregs, it
is a goal to observe an effect o f the reinforcement on the efficiency o f microwave
processing. Finally, the experimental data obtained will be applied to a pultrusion model
to begin the formulation o f a computer program that will assist in the future integration of
on-line monitoring equipment for the microwave pultrusion apparatus.
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Chapter 1. Background
1.1. Microwave Processing
Fiber reinforced polymer composites are widely used in the military, the aerospace
industry, and the sporting goods world. The primary way o f processing these composites
is thermally. The autoclave process requires a relatively long time to perform. To
increase processing speed and to treat thick thermosetting composites, industry has turned
to microwave processing. Microwave heating is an alternative method to conventional
thermal heating for the processing o f materials. Compared to the thermal heating method,
microwave curing o f composite materials is a faster and more direct heating m ethod that
takes advantages o f the dielectric properties o f the material being processed.
An electromagnetic field interacting with a dielectric material, such as a composite,
heats from the inside out. The temperature profiles inside the composite and the
absorption o f microwave energy are a strong function o f the electromagnetic wave modes
resonating inside the cavity. [ 1] By changing the length o f the microwave cavity and/or the
coupling probe depth, the waves are allowed to resonate inside the cavity, and the
reflected pow er is minimized. The cavity length controls what mode is excited and the
coupling probe, which attenuates the radio frequency signal, controls how much pow er is
reflected back into the circuit from the cavity. The general advantages o f microwave
heating are rapid, inside-out heating, better control o f the material's exotherm, selective
heating based on the lossy magnitude o f the constituents in a material, and improved
mechanical properties o f the composite.
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1.2. Microwave Heating Mechanism
In conventional thermal heating, the surface o f the material is heated and the
heating o f the interior occurs by thermal conduction and by the exotherm o f the reaction if
the polymer is thermosetting. On the other hand, microwaves heat the bulk o f the material
simultaneously and offer fast, selective, instantaneous, and relatively controllable heating.
Microwaves can penetrate into the bulk o f the material and cause heating by coupling with
the material at the molecular level. Microwave energy excites dipole groups, which align
themselves with the electric field. After the electric field is removed, there is relaxation o f
the dipoles, which causes heating at the molecular level. The dielectric constant is a
measure o f the material's ability to store electrical energy. The dielectric loss factor
(Equation 1) corresponds to the material's ability to dissipate electrical energy as heat.
The effective dielectric loss factor consists o f two terms: the loss factor due to the
dielectric and a term dependent on the conductivity and processing frequency. The
electrical conductivity portion o f Equation 1 is often large when a conductive material is
present, so as to drown out the dielectric term.
The materials' dielectric properties play an important role in determining how well
radio frequency heating will work. The dielectric loss factor is a function o f temperature
and extent o f cure. The dielectric constant and loss factor are both implicit functions of
temperature by way o f the relaxation time. The relaxation time is the time is takes for
dipolar molecules to reorient themselves after they have been aligned with the electric
field. The relaxation time is a function o f temperature through an Arrhenius relationship.
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The penetration depth, 8, in Equation 2, o f the electromagnetic waves is a function o f the
dielectric properties. [2]
^ " V V /2
1/2
5 =
2a
1+
-
1/2
(2)
V e'J j
The penetration depth, which is traditionally known as the point in the thickness o f
the sample where the power has decreased to 37% of its incident value, is inversely
proportional to the processing frequency. As frequency decreases, the skin depth
increases, meaning the waves travel further into the tape than would be observed with
higher frequencies.
The dielectric power absorption per unit volume o f the material can be calculated
using Equation 3, in which the power absorption is proportional to the dielectric loss
factor.
Pabs = 27r/e0e ’tan5EE
where
Pabs
/
e0
s’
E
tan5e
=
=
=
=
-
(3)
Power absorption per unit volume
Frequency (Hz)
permitivity o f free space (8.854 x 10'14 F/m)
Dielectric constant
Electric field strength
Dielectric loss tangent
The dielectric loss tangent is equal to the ratio o f s ’Vs’, so it is apparent that the effects of
microwave heating are predominant in materials where the dielectric loss factor is high
relative to the dielectric constant. The dielectric loss factor is a function o f the
temperature and frequency. For reactive systems, it is also a function o f the extent o f
cure. The absorbed power can be set into Equation 4 to generate basic theoretical
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temperature versus time curves. I f the percentage o f power absorbed is the same when all
o f the experimental conditions except input power level are kept the same, an increase in
input
dT/dt = Pabs / (Cp*p)
(4)
power would lead to an increase in power absorption per unit volume. T he slope o f the
temperature-time curves would then be directly proportional to the input pow er level. In
other words, increasing the microwave power will increase the slope o f th e temperature­
time curve, which is synonymous with a more rapid heating rate. Preliminary heating
studies on the composite material will attempt to prove this theory.
1.3. M icrow ave H eating A dvantages
There are numerous advantages to using microwaves for the processing o f
polymers and composites: decreased processing times, "inside-out" and selective,
molecular heating, control o f the exotherm by turning o ff microwave pow er, and improved
mechanical properties due to better resin/fiber interfacial bonding. Because thermal
heating o f composites heats from the outside-in, the material on the outside surface could
degrade before the middle o f a large-profiled part reaches the cross-linking temperature.
If the thermal conductivity o f the resin decreases during the cure cycle, it becomes more
difficult to transfer the necessary heat to cure the inside o f the part. Often, there is a post­
cure step when using conventional techniques, which increases processing times. Radio
frequency technology has been used in pultrusion lines for more than tw enty years in the
form o f a preheater for the wetted fibers. Often a large frequency sweep is used for this
preheating step. By installing a single-mode microwave applicator in the pultrusion line
5
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that operates at either a single frequency o r a series o f intelligently selected frequencies, it
is believed that processing times will decrease. By knowing the dielectric properties o f the
materials and how each frequency will directly and instantaneously interact with the
composite, microwaves can properly and efficiently heat the material from the inside-out
and at th e molecular level, thereby largely decreasing processing times.
F o r conventional processing, pultrusion dies have several temperature zones to
control the temperature and viscosity o f the material in the die. The zones also control the
gelation point inside the die. As the resin begins to cure, crosslinking occurs which rids
the molecules o f their dipolar nature. The microwave energy becomes selective in what it
heats in the resin/fiber mixture because only the monomer dipoles can align themselves
with the electric field, not the crosslinked polymer molecules. If the reinforcement
material has better electrical properties than the resin, the fibers will absorb the majority o f
the m icrowaves and help cure the resin at the interface through conduction. This transfer
o f heat at the resin/fiber interface [3] will improve the bonding, and subsequently, the
mechanical properties.
1.4. M icrow ave A pplicators
F our types o f applicators are prevalent in the microwave industry: multimode,
single-mode, waveguide, and specialized applicators. Multimode applicators are favorable
because they can excite several electromagnetic modes at a single frequency, and they are
able to process a wide range o f materials. On the downside, they are not particularly
energy efficient and can have trouble heating uniformly.
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Single-mode applicators heat m ore efficiently than multimode applicators. The
single electromagnetic mode excited in the applicator causes regions o f the process
material to be heated in a highly localized fashion. "The heating uniformity inside lossy
materials is a strong function o f the resonant mode." [4] I f medium- to low-loss materials
are processed in the applicator, there is no single mode that will heat the large sample
uniformly. [4] The single-mode and multimode applicators are used to process low-loss
and medium-Ioss materials.
Either single mode heating or mode switching heating can be used in the process.
Proper mode switching heating provides a m ore uniform temperature profile and can help
alleviate the problems o f processing low-loss materials in single-mode applicators. In
fixed frequency processing, adjusting the cavity length mechanically can change the
modes. With the variable frequency pow er source, the modes can be changed by
electronically tuning the frequency, which is a much better approach than mechanical
tuning.
The third applicator that can be used to process materials with microwaves is a
waveguide. It is a rectangular or cylindrical hollow pipe made o f a conducting material.
Materials are processed with a traveling wave, as opposed to a standing wave in the above
microwave applicator examples. Because the traveling wave is partially absorbed by the
material in the waveguide, the materials are often o f high-Ioss so the waveguides are not
inordinately long.
The final applicators used in industry are specialized applicators such as part­
shaped cavities or horn applicators. M icrowave energy is coupled into these units by way
o f an iris or a series o f irises. Part-shaped cavities are built to have the same dimensions as
7
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the desired part, but the mold walls are the applicator by which the microwaves are
attenuated. Horn applicators concentrate microwave energy before it is delivered into a
larger chamber. Although the design aspect o f the specialized applicators is more difficult,
the customer will save money on tooling costs if various dies are normally inserted into the
single- or multi-mode applicator.
1.5. Research Scope and Goals
The objectives o f this project are to obtain experimental data o f microwave
pultrusion processing and to compare variable frequency and single frequency processing
methods. There will be a fundamental study o f the microwave pultrusion equipment,
aimed at providing the data for a pultrusion system design. It is also a goal o f this project
to find the optimized microwave pultrusion variables o f input power and pulling speed.
The beginning o f this study includes system design and material selection. The
materials to be used in this study are continuous, unidirectional prepregs consisting of
glass fiber or graphite fiber and epoxy resin. The effect o f the reinforcement on
microwave processing efficiency will be investigated through heating studies and extent of
cure measurements. The prepreg was purchased from suppliers. Hexcel provided samples
o f their AS4/3501-6 prepreg tape. It is an amine-cured epoxy resin with unidirectional
graphite fiber reinforcements. [5] The glass reinforced epoxy prepreg, DA409/E250-6”,
was purchased from Adhesive Prepregs for Composites Manufacturers. [6] Several plies
o f the prepreg will be stacked together to form the composite prepreg tape for pultrusion.
Differential Scanning Calorimetry (DSC) will be used to determine the extent o f cure of
the pultruded samples. The pultrusion tests will be accomplished using the best heating
8
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mode for the prepreg system at a certain processing speed and input power. The Teflon
die will be used because o f its transparency to microwaves. The relationship o f
microwave processing speed, input power, and extent o f cure will be investigated to find
the optimum operating conditions for microwave pultrusion processing. Pultruded
samples are expected to exhibit good hydrolytic stability, improved mechanical strength,
and electrical insulation properties. Mechanical tests, such as flexural and tensile strength
measurements will be conducted. A variable frequency power source will be used to
determine if mode-switching heating can provide more even heating in pultruded
composites than with fixed frequency heating. Finally, the experimental data obtained will
be applied to a pultrusion model to begin the formulation o f a computer program that will
assist in the integration o f on-line monitoring equipment for the microwave pultrusion
apparatus.
9
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Chapter 2: Application of Continuous Microwave Processing
2.1. Introduction
M icrowave heating is an alternative method to conventional thermal heating for
the processing o f materials. Compared to the thermal heating method, microwave curing
o f composite materials is a faster and more direct heating method that takes advantages o f
the dielectric properties o f the material being processed. In conventional thermal heating,
the surface o f the material is heated and the heating o f the interior is induced by thermal
conduction. On the other hand, microwaves heat the bulk o f the material simultaneously
and offer fast, selective, instantaneous, and relatively controllable heating. Microwaves
can penetrate into the bulk of the material and cause heating by coupling with the material
at the molecular level. Microwave energy excites dipole groups and the relaxation o f the
dipole groups causes heating at the molecular level.
The continuous microwave processing technique is highly desired for processing
large composite materials that need even and fast heating such as long pipes or panels.
The shape o f the product is determined by continuously pulling the composite material
through a die to produce uniform profile parts. The idea o f continuous microwave
processing is to pass the material through the microwave applicator. The applications of
continuous microwave processing in industry have been used to preheat materials or to
post-cure parts. [7] Asmussen describes a general purpose microwave applicator that is
commercially available. [8] The unique feature o f this apparatus is that it allows for precise
tuning for mode selection and fine tuning within a mode selected. The problem, which
was not solved in using this apparatus, is how to seal the cavity when the material is being
continuously processed through the microwave cavity. The control of the microwave
10
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leakage from the entry and exit ports o f the applicator is one o f the key points in
continuous microwave processing. The microwave leakage during continuous processing
o f non-conductive materials has been controlled by attaching extended entry and exit ports
to the microwave cavity. [9] A method for controlling microwave leakage in pultrusion of
conductive materials has also been developed. [10]
One pultrusion application o f processing materials continuously in a waveguide has
been patented. [11] The idea o f using a waveguide as part o f a pultrusion die is excellent
in terms o f the simplicity o f the system. However, the dimension o f the waveguide has to
be in accord with the dielectric properties o f the materials being processed in order to
create the suitable microwave field patterns. [12] This requirement has greatly limited the
wide application o f the technique in industry due to the large variety o f materials being
processed. A microwave transparent die will be used for the microwave pultrusion
process. Heating and curing profiles o f the pultruded material will be studied at different
processing conditions. The current microwave pultrusion experimental apparatus [10,13]
developed at M SU will be the basis o f this pultrusion system study.
2.2. Pultrusion
The pultrusion process is a continuous manufacturing method, which can be used
to produce reinforced plastics with a constant cross-sectional area through the length o f
the product. The shape o f the product is determined by continuously pulling the
composite material through a die to produce uniform profile parts. Pultruded composites
consist o f reinforcing materials, a resin that binds the fibers together, and often a mat
material to improve the appearance of the com posite’s surface and other ancillary
11
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materials. [14] The key step in a pultrusion process is to control the interactions among
fiber, resin, and additives.
The general pultrusion process involves pulling continuous strands o f glass or
graphite fiber from a creel station. To alleviate the build-up o f static charge in the fibers,
the creel station is often in the form o f steel shelving to ground the fiber spools. A shelf
can hold up to 20 roving packages. [15] The fibers can be passed through ceramic rings to
prevent the fibers from crossing paths and causing entanglements in the resin bath. Fiber
mats or woven rovings can be added to the outside o f the part before the die to improve
the transverse strength. There is approximately 500 feet o f mat per roll. [15] Surface veils
are also used to give a more finished look to the finished part. Fibers move through a set
o f rollers at the entrance and exit of a resin bath to ensure complete wetting out o f the
fibers. This is the most important step in establishing good mechanical properties. Slower
line speeds and higher resin bath temperatures give better fiber wet-out results.
The resin bath is metal and contains a plug so it can be drained and cleaned. After
the resin bath, the fibers pass through a set o f rollers to rid the fibers o f excess resin.
Preformers before the die begin to align the wetted fibers into the desired profile. Cross
sectional wall thickness o f the part should help estimate the line speed needed to make a
satisfactory product. If the wall thickness is 3/16”, Vi-3/8”, or V2-3/4”, then the average
line speed should be 32, 20, or 16 inches per minute, respectively. [15]
Platens or electrical strip heaters heat the metal die. Platens, although easy to
assemble, are only able to heat the top and the bottom o f the die. They are usually placed
equidistantly down the die in the beginning, middle, and end. Platens are less efficient than
strip heaters, which will raise energy costs. [15] Zoned, electrical strip heating controls
12
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tem peratures along the width and the length o f the die. The pultruder will have more
control over the optimization o f the process parameters such as pulling speed and die
temperature. The conventional processing method is to heat the materials in the die
through thermal convection/conduction. Thermal heating is slow and requires a long die,
often causing uneven heating in large parts.
The preformers before the metal die are somewhat cool to prevent premature
gelation o f the resin/fiber system. The die is approximately three feet long. The resin
viscosity initially decreases upon entering the heated die. As the fibers conform to the
desired profile, the resin is allowed to further penetrate fibers. The die’s temperature is
increased to ensure that a resin skin does not form on the die walls, causing undue shear
forces and poor finishes as the rollers pull the part through the die. [16] In the industrial
pultrusion apparatus, the heating zones are monitored and controlled by an experienced
operator. Pulling speed has a lot o f influence on where the peak exotherm will be located
in the die. Ideally, one would like the pultruded part to be nearly cured before exiting,
thereby allowing it to transfer some o f its heat to the die walls. If the exotherm peak
occurs to close to the die exit, residual heat in the center o f the part could cause fractures
in the end product. [16]
The pulling section or mechanical pulling apparatus can be a set of rollers or a pair
o f continuous belts containing pads that grip the pultruded product. Terms used are
hydraulic reciprocating (easy set up), electromechanical caterpillar (requires a large
number o f pulling blocks), and ballscrew (high maintenance). [15] In a commercial
process, pulling speeds can range from 2-3 in/min to 10-15 ft/min. [16] If maximum
pulling speeds are desired, it is best to have multiple dies in the line so as to avoid
13
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incomplete fiber w et-out and curing. [16] The pulling force must be greater than the
frictional force o f the fibers and the shear force o f the resin layer against the die wall and
the fibers dragging against the resin flowing back into the bath. [16]
A cut-off saw, wet or dry and automatic or manual, provides a method to produce
the pultruded members at the customer's specified length requirements. A wet saw has
water that cools and lubricates the blade at the same time it prevents harmful fiberglass
dust to travel through the air. A dry saw needs a dust collection system. Automatic saws
are programmed to cut at the appropriate lengths, while manual saws are run by the
operators themselves. [15] Cut parts are then sorted and stacked for finishing, packaging,
or shipping.
2.3. Experimental Setup
The experimental apparatus (Figure 1) at Michigan State University [10,13]
includes a pow er source that is either a single frequency (2450 MHz) source with a
magnetron or a variable frequency (1700-4300 M Hz) power source with a microwave
power generator and TWT (traveling wave tube) amplifier. Circuit components include
directional couplers to send power to the cavity and to the incident and reflected power
meters, circulators to prevent any reflected power o f damaging the power source, coaxial
connectors, coaxial cables, and resistors for dummy loads. Power meters were used to
measure the incident and reflected power, and Luxtron temperature units were used to
measure the surface temperature of the prepreg as it was pulled through the cavity.
14
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Coupling Probe
Circulator
Directional
Coupler
Sweep
Oscillator
Microwave
Source
Dummy
Load
Oscilloscope
Pr
PC
A/D
Figure 1. Microwave Pultrusion Experimental Apparatus at M SU
2.3.1. Microwave Cavity
A Teflon die, as shown in Figure 3, which is transparent to microwaves, producing
a profile measuring 3.80 cm wide by 0.294 cm high was inserted into an opening in the
side o f the 17.78 cm inner diameter, tunable, cylindrical, batch, brass microwave cavity.
The dies need to have sufficient dimensional stability and mechanical strength, just like the
ones used in conventional pultrusion. Ceramic dies are also transparent to microwaves.
Teflon dies have lower dielectric loss than the ceramic dies, but have less mechanical
strength. The cavity can be tuned for mode selection and fine tuned to minimize reflected
power by mechanically adjusting the sliding short height and coupling probe depth,
respectively. The microwave cavity in Figure 2 is modified for pultrusion processing such
that doubly corrugated chokes [9] are extended from the cavity to contain the
15
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electromagnetic radiation. Conductive fins [3] are grounded to the cavity opening and in
turn, ground the induced current in the conductive graphite fibers when graphite/epoxy is
processed. Finger stock placed in the opening out o f the jacket boxes provides additional
dampening o f the induced current, thereby reducing microwave leakage.
'■*,4.
f \'/*'}
SLIDING SHORT
Lc
Coupling Probe
Lp
Figure 2. Schematic o f Microwave Applicator at MSU
Figure 3. Entrance To Microwave Cavity through Teflon Die
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2.3.2. Calibration of Rollers
A Penta Pow er m otor and a Baldor motor were chosen to pull the prepreg tape
through the system. The m otors’ dial settings were calibrated to calculate possible
processing speeds (see Table 1). Past work at M SU suggested that the mechanical pulling
system be improved to allow for increased product throughput. [17] It was found that the
maximum obtainable processing speed was 4 cm/s (7.875 ft/min) with the Baldor motor.
Table 1. Calibration o f Rollers to Evaluate Possible Processing Speeds
Dial Setting Penta P ow er S p eed (cm /s) (ft/min) Baldor S p eed (cm /s) (ft/min)
20
0.143
0.281
0.400
0.787
25
0.423
0.215
30
0.286
0.562
0.870
1.712
35
0.373
0.733
40
0.451
0.887
1.333
2.625
45
0.527
1.038
50
0.594
1.170
2.143
4.218
60
0.667
1.312
2.857
5.624
70
0.800
1.575
2.857
5.624
80
1.000
1.969
4.000
7.874
90
1.333
2.625
4.000
7.874
100
1.333
2.625
4.000
7.874
2.4. Materials: Preparation and Discussion
The materials used in the study were a Hercules AS4/3501-6 graphite/epoxy
composite and a DA409/E250 glass/epoxy composite. The reason for performing
experiments on both systems is to compare and contrast the ability to process each
material. Processing speeds and efficiency o f microwave heating are important factors
that are influenced by the type o f reinforcement that is used. The electrically conductive
nature o f the carbon fiber initially suggests that it will absorb more o f the electromagnetic
radiation and dissipate the heat throughout the composite by conductance. Glass fibers
are less electrically conductive, but are inherently denser than the carbon fibers, leading
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one to believe that they may provide better mechanical properties. An optimum product
could involve a mixture o f these two reinforcements.
As was discussed before, the dielectric constant is a measure o f the material's
ability to store electrical energy. The dielectric loss factor corresponds to the material's
ability to spread electrical energy as heat. The effective dielectric loss factor consists o f
tw o terms: the loss factor due to the dielectric and a portion that deals with the
conductivity. The materials' dielectric properties play an important role in determining
how well radio frequency heating will work. The dielectric loss factor is a function of
temperature and extent o f cure. The dielectric constant and loss factor are both implicit
functions o f tem perature by way o f the relaxation time. The relaxation time is the time is
takes for dipolar molecules to reorient themselves after they have been aligned with the
electric field.
The University o f Mississippi Pultrusion research group conducted pultrusion
experiments on an epoxy system with glass fiber and carbon fiber reinforcements. [18]
Using a thermally heated die, the temperature and extent o f cure profiles across the die
were very similar for both material systems. When pultruding a composite using thermal
techniques, the resin's thermal conductivity is an important variable in determining the cure
inside the composite. The cured material on the outside o f the pultruded part continues to
be heated for the sole purpose o f making sure the exotherm in the center o f the part is
enough to meet cure specifications.
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2.5. Goals
The objectives o f this project are to obtain experimental data o f microwave pultrusion
processing and to compare variable frequency and single frequency processing. It is
thought that variable frequency will provide a more even temperature distribution across
the width o f the prepreg tape. It is also a goal of this project to find the optimized
microwave pultrusion variables o f input power and pulling speed. Extent o f cure and
mechanical property (tensile and flexural strength and modulus) measurements will be
made to support process design. Comparison and contrasting o f the effect o f fiber
reinforcements on the efficiency o f microwave processing will be made. Possible on-line
monitoring techniques will be investigated for future study.
19
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Chapter 3: Cavity Characterization
3.1. Finding Electromagnetic Modes
The brass cavity was characterized to obtain the resonant modes by mechanically
changing the cavity length and the coupling probe depth. Normal modes in a right circular
cylinder consist o f transverse electric (TE) and transverse magnetic (TM) modes.
Subscripts define which TE-mode is resonant in the cavity: 1 = number o f full-period
variations o f Er with respect t o 9 , m s number o f half-period variations o f E0 with respect
t or, n s number o f half-period variations o f Er with respect to z. The subscripts are
defined equally for the TM-modes, but E is replaced by H, the magnetic field. Equation 6
shows the definition for the resonant frequencies inside the cavity. [19]
(6)
where c = speed o f light = 3.00 x 108 m/s
xim = m1*1root o f Henckel Function J'i(x) = 0 for the TE-modes
xim =
root o f Bessel Function Ji(x) = 0 for the TM-modes
From the equation above, a mode chart was constructed for the 7" brass cavity in
which the frequency in Hz was plotted versus the cavity length in centimeters. It was
assumed that the mode charts were independent o f coupling probe depth, which assists in
minimizing reflected power. It was also assumed that the chokes on the entrance and exit
ports o f the cavity did not affect the electromagnetic waves inside the cylindrical cavity
itself. The Teflon o r ceramic die, albeit transparent to microwave radiation, was assumed
not to interfere with the resonant conditions o f the cavity. There was no material inserted
into the cavity o r the die. The x-axis is drawn from the 2.45x109 Hz point on the y-axis
20
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because that was the processing frequency used by the single frequency pow er source. As
seen by Figures 4 and 5, one may obtain eight different modes in single frequency
processing by changing the cavity length a range o f 10.5 cm.
6.0000E+09
5.0000E+09
—
—
4.0000E+09
TE011
TE012
TE111
—* - T E 1 1 2
H 3.0000E+09
—* —TE211
—
TE212
— I— TE312
TE311
2.0000E+09
1.0000E+09
CO
co
o
CM
CO
co
Cavity Length, Lc, cm
Figure 4. Frequency vs. Cavity Length for 7.00" +/- 0.001" (TE modes)
21
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6.0000E+09
5.0000E+09
TM011
N
X
4.0000E+09
7M 010
TM110
- x — TM210
o 3.0000E+09
TM111
cr
S'
TM012
2.0000E+09
- i— TM 112
— — TM211
1.0000E+09
TM 212
0.0000E+00
6
7
8
9
10
11
12
13
14
15
16
C av ity L e n g th , Lc, cm
Figure 5. Frequency vs. Cavity Length o f 7.00" +/- 0.001" (TM modes)
If one chooses a fixed cavity height o f 15.5 cm, eleven modes can be excited over
the 2.00 to 3.00 GHz frequency range. Electronic tuning o f frequency is faster than the
mechanical tuning o f the cavity, giving rise to the idea o f variable frequency processing o f
composite materials. Heating studies were conducted using a processing frequency o f
2.45 GHz for each mode to find which electromagnetic mode most efficiently heated the
material.
3.2. Temperature Measurement
Fluorotropic tem perature probes, which are transparent to microwaves, were
placed through holes in the die 8 mm apart across the width o f the die and 21 mm apart
along the length o f the die (Figure 6). The probes rested against the surface o f the
22
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prepreg. Temperature readings were sent from the Luxtron temperature meter to a
personal computer for data collection and analysis.
5.7 cm
8 mm
4.9 cm
•
21 mm
24.6 cm
Figure 6. Teflon Die Used for Heating Studies and Microwave Pultrusion Processing
3.3. H eating Studies A t V arious Modes
Heating studies were conducted using a processing frequency o f 2.45 GHz for each mode
to find which electromagnetic mode most efficiently heated the material. Table 2 lists the
cavity dimensions used for the characterization studies. The Teflon die was in place when
the preliminary cavity dimensions were measured. Tables 3 and 4 list the heating modes
for the 17.78 cm cavity with the six-ply graphite/epoxy and glass/epoxy prepreg inserted
into the cavity, respectively.
Temperature versus time curves are shown in Figures 7 and 8 for the
graphite/epoxy and glass/epoxy systems, respectively. The glass/epoxy system was not
pultruded for the full range o f process variables in the heating study because the
temperature rise was small over time, and an elevated temperature value was desired to be
able to choose the proper mode to continue studying.
23
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Table 2. Heating Modes for the Cavity with Teflon Die
M ode #
1
2
3
4
5
M ode T ype
TM 011
TE O il
TM 111
TE 112
TE 311
C avity L ength, Lc, mm
70.5
100.5
119
131
152
P ro b e D epth, L p, m m
8.80
25.29
13.15
6.95
13.32
Table 3. Heating Modes for the Cavity with Teflon Die and Graphite/Epoxy Prepreg
Mode#
1
2
4
5
M ode T ype
TM 011
TE 011
TM 111
TE 112
TE 311
C avity L ength, Lc, mm
70
99
115
133.5
151
P ro b e D epth, L p, mm
8.80
16.17
21.57
23.42
13.49
Table 4. Heating Modes for the Cavity with Teflon Die and Glass/Epoxy Prepreg
Mode#
1
2
->
4
5
M ode T y p e
TM 011
TE 011
TM 111
TE 112
TE 311
C avity L ength, Lc, mm
70
101
117
131
152
P ro b e D epth, L p, mm
11
13.94
15.75
9.5
16.33
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80
70
60
50
TED11
TM111
40
TE112
30
TMD11
TE311
20
10
0
50
100
200
150
250
300
-10
tim e , t, s e c
Figure 7. Temperature Versus Time Results for Heating Studies o f Six-Ply Hercules
AS4/3501-6 Prepreg at 100 W atts and 0.787 ft/min
50
45
40
35
TE011
30
TM111
25
TE112
TMQ11
20
TE311
15
10
5
0
100
300
200
400
tim e .t, s e c
Figure 8. Temperature versus Time Results for Heating Studies o f Stationary Six-Ply
DA409/E250 Prepreg at Approximately 50 Watts
25
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3.4. Preliminary Heating Studies In TEm Mode
Three foot, six-ply Hercules AS4/3501-6 prepreg tape samples were processed in a
T E m mode cavity at 2450 MHz in the existing pultrusion apparatus with input powers o f
50, 75, and 100 W atts and a pulling speed o f 0.281 ft/min. From the results in Figure 9, it
was observed that an increase in input power increased the steady state temperature value
o f the material. Also, the linear portion o f the temperature vs. time curve has a larger
slope for increasing input power, suggesting that higher power leads to decreased
processing times. Table 5 lists the relative values from the extent o f cure measurements
conducted on DSC.
70
60
50
O 40
o.
|
30
20
0
100
200
300
400
500
600
time, t, sec
Figure 9. Results For Preliminary Heating o f Six-ply Graphite/Epoxy Composite In T E m
Mode At Various Input Powers
26
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Table 5. Extent o f Cure Measurement for Preliminary Heating o f Six-Ply Graphite/Epoxy
Composite in T E u 2 M ode with Input Pow er Varied
Input Power (Watts)
Pulling Speed (ft/min)
Extent of Cure
50
0.281
21.97%
75
0.281
96.66%
100
0.281
97.74%
3.5. Conclusions
The continuous microwave applicator was characterized to find which
electromagnetic modes could be excited during pultrusion. Mode charts for a 7" brass
cavity gave promising prospects for variable frequency processing because eleven modes
could be excited at a specified cavity height. Heating studies o f the two prepreg materials
gave insightful results in establishing what mode heated the material most efficiently.
TEi 12 mode was chosen for the fixed frequency studies.
It appears that both the graphite/epoxy and glass/epoxy systems are able to be
processed in the microwave environment. It is shown in Figures 7 and 8 that there is a
more rapid temperature increase in the graphite/epoxy prepreg than the glass/epoxy
prepreg for the same heating mode. Graphite fibers absorb more electromagnetic energy
than the epoxy. In glass/epoxy systems, the matrix absorbs the majority of the energy.
There was a slower rate in temperature rise for the glass/epoxy because there was more
reflected power during processing. The energy was coupling efficiently with the
composite. The microwave cavity must be finer tuned w hen processing the glass/epoxy
system. Preliminary pultrusion experiments on the graphite/epoxy prepreg revealed that
27
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the surface temperature rises more rapidly and to a higher temperature with an increase in
input power, supporting the hypothesis that higher microwave input pow er will lead to a
decrease in processing time. Promising data collected in this series o f experiments set the
stage for more in-depth fixed frequency and variable frequency studies.
28
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Chapter 4: Fixed Frequency Pultrusion Experiments
4.1. Background
Three foot long, 1 X
A" wide, six-ply Hercules AS4/3501-6 prepreg tape samples
were processed in a T E m mode cavity at 2450 MHz in the existing pultrusion apparatus.
Input powers o f 40, 45, 50, 55, 60, 65, 70, 80, and 100 Watts were used to heat the tape
at a pulling speed o f 0.281 ft/min. Pulling speeds with the corresponding dial settings o f
20, 25, 30, 35, 40, 45, and 50 on the Penta Power m otor (0.281 ft/min - 1.17 ft/min) were
used to process the tape while an input power o f 100 Watts was selected.
From the heating studies, the T E m mode was established as a center heating mode
where the electric field was the strongest along the central, axial (pulling direction) down
the tape (Figure 23). Optimizing the pultrusion variables o f pulling speed and input power
is important in producing a high quality part. Even though high product throughput is
desired, good resin/fiber adhesion is paramount in achieving the mechanical properties
sought by the customer.
4.2. Experimental Setup
Six plies o f three foot by 1 W prepreg were cut parallel to the continuous fiber
direction. Next, they were stacked and sent through a pressurized roller to rid the tape o f
any voids and air bubbles that were present from the stacking process. The tape was
manually fed through the Teflon die in the side o f the cavity until just before the pulling
rollers. The circuit was closed by connecting the input coaxial line to the 100 W att fixed
frequency pow er source and the output line to the coupling probe. The Luxtron
tem perature probes were placed across the die width in four locations and calibrated to
29
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ambient conditions. The probe tips touched the surface o f the prepreg for accurate
temperature measurement. Temperature data was collected by electronic signals sent to a
PC. Temperature measurements began for a 30 second period to establish a steady state
reading. After the 30 second start-up, the pulling rollers and power source were
simultaneously turned on to their pre-determined positions. The reflected power meter
was monitored to insure that maximum power was being delivered to the composite.
Temperature readings halted once the back end o f the prepreg exited the cavity. At this
time, the pow er source was turned off. Upon exiting the cavity, the tape traveled through
a set o f pressurized rollers. The pultruded product was left to cool before thermal and
mechanical analysis samples were made and tested.
4.3. V ariation in Pulling Speed
At a fixed pulling speed, the material's residence time in the cavity defines how
long the microwave energy has to heat the product to its curing temperature. If the
pulling speed is too fast for the power input level used, the material will exit the cavity
partially cured and a post-curing step will be needed. On the other hand, too slow of a
pulling speed may cause the resin to be overheated and degrade. "The location o f the
exothermic peak depends on the speed of pulling the fiber-resin stream through the die."
[16] The exotherm peak will shift towards the die exit with an increase in pulling speed.
This will also increase the pulling force needed by the mechanical systems. By combining
temperature versus time curves and extent o f cure measurements, one can determine the
point where the exotherm has ended and continuing with the addition o f microwaves could
cause undue heating. The only chance for heat transfer when the tape has exited the
30
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cavity is from the heat conduction through the resin and fiber, which tends to be low, and
by the remaining exotherm o f the polymerization reaction. It is desired that the exotherm
be completed before the die exit because if the interior o f the pultruded part is still at an
elevated temperature, interlaminar cracks may form within the product. [16]
4.4. Variation in Input Power
The issue o f input pow er level must be considered. In the microwave pultrusion
energy balance (see Chapter 7), input power is an implicit variable in the heat generation
due to the power absorption term. In a published power absorption model [1], the five
parameters o f interest are all explicit functions o f input power. Also, to shorten
processing times and times for a material to reach a desired temperature, higher power
input is required, as suggested by the preliminary heating studies on the graphite/epoxy
prepreg. At a fixed pulling speed, too high o f power may cause the resin to degrade and
the mechanical properties may suffer. Not enough power will result in a low-cured
product and the resin/fiber interfacial adhesion will be poor. Extent o f cure and
mechanical property measurements will be related to pulling speed and input power in
order to find the optimum processing conditions.
4.5. Extent of Cure Measurement (Differential Scanning Calorimetry)
Because T E i I2 mode was used, the magnitude o f the electric field was greater at
the center o f the cavity and parallel to the pulling direction. Further examination o f the
prepreg revealed it to be preferentially heated down the center o f the tape. Relative values
o f extent o f cure were found using DSC. A sample from an uncured piece o f
31
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graphite/epoxy composite was heated 10°C/min to 350°C to establish a theoretical
maximum heat o f reaction. When an epoxy matrix is heated, 100% o f the possible
reactive sites do not react, so 100% extent o f cure is not theoretically possible. Extent of
cure values reported below and in plots in Figures 10 and 13 are relative values and not
absolute. Samples were taken from the center o f the prepreg tape. Samples were heated
10°C/min to 350°C to find the heat o f reaction, which was compared to the heat o f
reaction for the uncured composite. Equation 7 shows how the extent o f cure was
calculated.
H rc
a =l- ~ fL
(7)
RU
where a= extent o f cure
HRc=heat o f reaction o f cured sample (J/g)
HRU=heat o f reaction o f uncured sample (J/g)
The heat o f reaction for the uncured composite sample was found to be 197.5 J/g.
Extent o f cure increased with microwave power input up to 70 Watts and then leveled off
to approximately 98%. At these high levels o f power input, the resin may have degraded,
resulting in what looks to be a "well-cured" product. There was negligible change in
extent o f cure with powers above 70 Watts. Powers below 40 Watts, which gave 18%
extent o f cure, had low heating potential and were neglected. Equation 8 simply reveals
why increasing input microwave pow er at the same pulling speed results in a higher extent
o f cure.
t =
(C p/ * wf + Cpm * ( l - wf )) * (Tc - Ta) * ( wm + wf )
“
32
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(o)
Above, the time, t, for the temperature to rise to a curing temperature, Tc, from the
ambient temperature, Ta, is dependent on the weight fractions, wmand Wf, and heat
capacities, Cpm and Cpf, o f the matrix and fiber, respectively. The microwave power, P, is
inversely proportional to time so an increase in power leads to a decrease in time. If the
material's residence time is the same for varying input power, the exiting temperature o f
the prepreg will be greater with increasing input power. The extent of cure is indirectly
related to the tem perature through the kinetic parameters in the kinetic model. Equation 8
is a basic energy balance, which assumes that all the incident power delivered to the
microwave cavity is absorbed by the composite, causing heat generation within the
material from dipole relaxation.
Extent o f cure increased somewhat linearly with increasing microwave power up
to 70 Watts and then leveled off to approximately 98%. The increase in power increases
the amount o f heat generation inside the composite. Heat conduction is low perpendicular
to the fiber direction, resulting in a low-cured outer region o f the product. There was
negligible change in extent o f cure with powers above 70 Watts. At a pulling speed o f
0.281 ft/min, power levels below 40 Watts had low heating potential and were neglected.
Figures 11 and 12 show trendlines with equations for extent o f cure as a function of input
power at a constant pulling speed.
33
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0.9 |
0.8 4O
®
§
i2
0.6 f
0 .5 0.4 —
0.3 -0.2 0.1
-
80
100
Input Pow er (W)
Figure 10. Effect o f Input Power on Extent o f Cure
0.8
-
0.7 -
y = 0.026x - 0.8687
R2 = 0.9887
0 .6 -r
0.5 0.4 0.3 0.2
-j-
20
30
40
70
Input Pow er (Watts)
Figure 11. Trendline for Extent o f Cure as Function o f Input Power (40-70 Watts)
34
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0.98
0.975 0.97 r
= 5E-05X2 - 0.0071 x + 1.1924
0.965 |
0.96 4
0.95 4
0.945 4
0.94
0.935 4
40
60
80
100
Input P ow er (Watts)
Figure 12. Trendline for Extent o f Cure as Function o f Input Power (70-100 Watts)
For pulling speeds between 0.5 ft/min and 0.7 ft/min, the extent o f cure decreases
rapidly. For pulling speed ranges o f 0.3-0.5 ft/min and 0.7-1.2 ft/min, the extent o f cure
changed only 15% in each range. It is suggested to operate within these ranges for greater
control o f extent o f cure and better quality o f product. It should be noted that line speeds
would increase with higher input power. The extent o f cure dependence on pulling speed
is justified by the "residence time" o f a differential slice o f prepreg inside the cavity.
Figures 14, 15, and 16 show the extent o f cure as a function o f the pulling speed and give
trendlines for each o f the three regions o f interest: the high cure section o f slow
processing speeds, the linear region o f moderate pulling speeds for a low input power, and
high pulling speed region below the gel conversion point. Longer residence times will
allow the electromagnetic energy to interact with the composite for a longer period o f
time. For slower processing speeds, the prepreg was cured across a greater width o f the
tape. For processing conditions that produce a product with an extent o f cure o f 30% (gel
35
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conversion for epoxy resin) or below, the microwave applicator could act as a preheating
device in a pultrusion line. If maximum extent o f cure is desired, the microwave
pultrusion apparatus could be the sole heating element in the pultrusion line.
0.9 40 . 8 -r
0.7 3 0. 6
--
® 0.5 -I
i
0.4
0.3 t
0.1
0.000
0.500
1.000
1.500
Pulling S p eed (ft/min)
Figure 13. Effect o f Pulling Speed on Extent o f Cure
0.9 -0.8
--
0.7 -0.6
--
y = -2.5999x + 1.6135x + 0.7293
S 0.4
0.2
-
0.000
0.200
0.400
0.600
pulling sp eed (ft/min)
Figure 14. Trendline for Extent o f Cure as Function o f Pulling Speed (0.281-0.562 ft/min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.800
0.700 0.600 t
£ 0.500 y = -1.5552x + 1.607
° 0.400 r
0.200 4
0.100 i
0.000 -i—
0.000
0.200
0.400
0.600
0.800
1.000
pulling sp eed (ft/min)
Figure 15. Trendline for Extent o f Cure as Function o f Pulling Speed (0.562-0.733 ft/min)
0.35
0.3
0.25 -
y = -0.6308X + 0.8468X + 0.056|
R2 = 0.9938
0.1
0.05 4
0.000
0.200
0.400
0.600
0.800
1.000
1.200
pulling sp eed (ft/min)
Figure 16. Trendline for Extent o f Cure as Function o f Pulling Speed (0.733-1.17 ft/min)
37
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4.6. Mechanical Properties Measurements
4.6.1. Tensile Strength
Tensile coupons measuring approximately 17.78 cm (7") long and 0.9525 cm
(0.375") wide at the middle were cut from the cured, unidirectional graphite/epoxy
product. Three samples were tested for each set o f processing conditions. The coupon
was shaped to specifications with a router while the sample was in a tensile bar metal
clamp. Tensile strength measurements were made using a pulling rate o f 1.27 cm/min
(0.5"/min), and the samples were pulled parallel to the fiber direction. A 1000 lbf load cell
was used, in which the grips each held one inch o f the tensile bar's ends. The load was
plotted against the strain to help calculate the tensile modulus. Three samples were made
for each processing condition. Coupon thickness and width in the middle were measured
at three points and averaged. Maximum tensile strength and load were measured and
tensile modulus was calculated through analysis o f stress versus strain curves.
The results for the tensile tests were averaged for the three samples, and those
values are shown in Figures 17 through 20. Measurements typically varied from the
average by no more +/- 10%. Values for the tensile strength ranged from 138 to 188
MPa. The tensile strength was not affected by pulling speed or input power variations
because the samples were tested by pulling in the fiber direction. The tensile modulus was
calculated by performing a linear regression on the initial portion o f the stress versus strain
curve. Average tensile modulus values ranged from 1100 to 1500 MPa, with values
varying +/- 15% from the average. Any anomalies in the curves were from experimental
error, such as the sample slipping in the tensile bar grips.
38
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200
180
160
140
120
100
80
60
40
20
20
40
60
80
100
Input Power (Watts)
Figure 17. Tensile Strength o f Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 ft/min and Variable Input Pow er
1400.00
1200.00
1000.00
800.00
600.00
400.00
h200.00
0.00
20
80
100
In p u t P o w e r (W atts)
Figure 18. Tensile Modulus o f Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 ft/min and Variable Input Pow er
39
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180
160
140
CL
120
as
100
80
60
c
40
20
0
0.2
0.4
0.6
0.8
1
1.2
!
Pulling S p e e d (ft/m in)
I
Figure 19. Tensile Strength o f Six-ply Graphite/Epoxy Prepreg Tape with an Input Power
o f 100 Watts and Variable Pulling Speed
1600
1400
"S' 1200
Q.
s
■3oo
2
01
1000
800
600
400
200
0
0.2
0.4
0.6
0.8
1
1.2
j
Pulling S p e e d (ft/m in)
Figure 20. Tensile M odulus o f Six-ply Graphite/Epoxy Prepreg Tape with an Input Power
o f 100 Watts and Variable Pulling Speed
For unidirectional composites containing fibers parallel to the pulling direction, the
samples fail by tensile rupture o f the fibers. [16] This is followed or accompanied by the
debonding o f the fiber/matrix interface parallel to the fibers. [16] Any final failures in the
composite are equal to (l-f)crrau, where f is the volume fraction o f fibers in the composite,
40
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and a mu is the ultimate strength o f the matrix. If the matrix begins to fail while the fibers
are still bearing a portion o f the load, then the composite's ultimate tensile strength can be
calculated from Equation 9. [20]
CTiu = f o f u + ( 1 -O tfm fu
(9 )
where cr^ = ultimate strength o f the fiber
CTiu = ultimate strength o f the composite
= matrix stress at the onset o f fiber cracking
4.6.2. Flexural S trength
Flexural strength tests were conducted on the Universal Testing System in
compliance with ASTM Standard D 790 - 84a. [21] The flexural strength and flexural
modulus were obtained for the tests. The support span-to-depth ratio was 16 to 1 for the
three-point loading machine. The pultruded composite samples were approximately 3.2
mm thick. Again, three samples were cut for each processing condition. The
recommended dimensions for test specimens were 25 mm wide and 80 mm long. The
support span was 50 mm, and the rate o f cross-head motion was 1.3 mm per minute.
Figures 21 through 24 show the averaged results for the flexural test
measurements. Values were averaged for the flexural strength and ranged from 25 to 120
MPa. Measurements typically varied from the average by no more +/- 10%. The flexural
strength o f the material is thought to be the maximum fiber stress at failure on the tension
side o f a flexural sample. [16] The flexural strength increased with increasing input power
and reached a maximum at approximately 60 Watts. At input powers greater than 60
Watts, the flexural strength decreased due to the brittleness o f the specimens. Flexural
41
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strength increased with increasing pulling speed up to a maximum o f about 0.42 ft/min.
For faster pulling speeds, the specimens bent much more because the resin/fiber interfacial
strength was low and the resin was undercured. The flexural strength values for these
higher pulling speed experiments were a reflection o f the non-homogeneity o f cure across
the width o f the prepreg.
Figure 21. Flexural Strength o f Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 ft/min and Variable Input Pow er
1400
5
1200
1000
w
S
800
600
400
200
0
20
40
60
80
100
Input Power (Watts)
Figure 22. Flexural Modulus o f Six-ply Graphite/Epoxy Prepreg Tape with A Pulling
Speed o f 0.281 ft/min and Variable Input Power
42
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60
50
40
CD
30
20
“■ 10
0.2
0.4
1.2
0.8
0.6
Pulling S p e e d (ft/m in)
Figure 23. Flexural Strength o f Six-ply Graphite/Epoxy Prepreg Tape with An Input
Pow er o f 100 Watts and Variable Pulling Speed
700
600
500
400
UJ
300
200
100
0.2
0.4
0.6
0.8
Pulling S peed (ft/min)
Figure 24. Flexural Modulus o f Six-ply Graphite/Epoxy Prepreg Tape with An Input
Power of 100 W atts and Variable Pulling Speed
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Average flexural modulus measurements ranged from approximately 300 MPa to
1400 MPa, with values varying +/- 15% from the average. The modulus suffered from a
low-cured product at low input power levels or too high pulling speeds and from a brittle
product with a partially degraded matrix. When fibers are long and continuous, there is
larger statistical chance that there will be flaws in the fibers which could allow for failure
in the composite. [22] The resin acts like glue between the sensitive, dry fiber bundles.
When a load is applied, the resin increases the fibers' ability to carry that load by holding
together the surface imperfections in the fibers. [22] If the resin degrades with
overheating, the resin/fiber interfacial bond strength is at risk. The resin will not fill its
role o f being glue and may become brittle just like the dry fiber bundles. It is important to
balance the processing conditions to ensure that the resin/fiber interface is strong, so the
resin can help improve the composite's strengths and moduli.
44
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4.7. Conclusions
Extent o f cure o f the graphite/epoxy composite was established as a function o f
input power and pulling speed for a microwave pultrusion apparatus. Extent o f cure
increased with an increase in microwave power input and decreased with an increase in
pulling speed. At 100 Watts input power and a pulling speed o f 0.281 ft/min, the center o f
the six-ply graphite/epoxy prepreg was 98% cured.
Tensile strength and modulus were relatively unaffected by the change in process
variables. There was variability in the flexural strength and modulus with respect to input
power and pulling speed. Too high o f power or too slow o f pulling speed resulted in a
brittle matrix. Low input power and fast pulling speeds gave a product that had a
relatively uncured matrix and low interfacial bond strength.
45
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Chapter 5: Variable Frequency Pultrusion Experiments
5.1. Background
Either a fixed frequency or variable frequency microwave pow er source can be
used to obtain uniform and fast microwave heating. When applying fixed frequency
heating, one must rely on the thermal properties o f the materials to dissipate heat
throughout the composite for a more even distribution o f cure. During the single mode
heating studies, the prepreg tape was not heated evenly over the width. Some transverse
electromagnetic modes revealed that the tape was heated more in the center (Figure 25),
while others heated preferentially on the outside edges (Figure 26). Temperature
gradients for the single m ode experiments reached as much as 40-45°C. Past results have
shown that an even cure distribution occurred after 15 minutes o f single mode heating.
[17] These results and the need to increase process throughput led to the idea of variable
frequency processing o f the prepreg, such that the cavity length would be kept constant
and processing frequencies would be intelligently selected.
Either single mode heating or mode switching heating can be used in the process.
Proper mode switching heating provides a more uniform tem perature profile. In fixed
frequency processing, the modes can only be changed by adjusting the cavity length
mechanically. With the variable frequency pow er source, the modes can be changed by
electronically tuning the frequency, which is a much better approach than mechanical
tuning.
In the variable frequency study, the 17.78 cm cylindrical brass cavity was
characterized, with a Teflon die and processing material inserted, for a frequency range o f
2.00 to 3.00 GHz. Heating studies for selected frequencies helped to intelligently select
46
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those used in further processing studies. Prepreg thickness was varied to investigate if
sample size affected microwave pow er absorption efficiency. Time intervals for each
processing frequency were varied to establish an optimum set o f processing conditions for
a particular prepreg thickness.
Figure 25. Six Ply Hercules AS4/3 501-6 Heated in T E m mode at 100 Watts and 0.281
ft/min
Position 3 corresponds to the middle o f the die, while positions 1 and 5 correspond
to the outer edges o f the die. Each position is 8mm apart. The y-coordinate is the
processing time in seconds and can be compared to die length in conventional pultrusion
processing by considering a differential slice o f prepreg and monitoring its temperature as
47
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it passes through the heater. Then, by multiplying the processing time by the pulling
speed, die length would vary from two to three feet for the cases below.
Figure 26. Six Ply Hercules AS4/3501-6 Heated in TEon mode at 100 Watts and 0.281
ft/min
5.2. Frequency Sweep Analysis
For an initial set o f cavity dimensions, the coupling probe was set at a depth o f
15.89 mm and the cavity length (Lc) was varied from 160 mm to 70 mm by 10 mm
increments. A Lambda variable frequency power source acted as an amplifier to a HP
Sweep Oscillator to enable the microwave input power to be increased to approximately
150 Watts. A process control program written by Qiu [23] measured the reflected power
48
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for a frequency sweep o f 2.00 to 3.00 GHz. Figure 27 is presented as an example o f a
poor selection o f cavity dimensions such that there are only a couple o f frequencies that
have below 10% reflected power. When high input power is desired, there is a greater
risk o f the source shutting down due to the reflected power, even at 10-15% reflection.
0.6
0.5 -
0.4
w
0)
5o
Q_
■ao
ts
nj
-
0.1
2
[i
2.1
2.2
2.3
2.5
2.4
2.6
2.7
2.8
2.9
3
-
frequency, f, GHz
Figure 27. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 90 mm And dp = 15.89 mm And Containing Sixply Hercules AS4/3501-6 Prepreg
Frequencies with minimum reflected power were individually selected to determine
their heating potential (Figure 28). A cavity length o f 135mm and a coupling probe depth
o f 15.89mm were used in the first study o f variable frequency processing o f a six-ply
Hercules AS4/3501-6 prepreg tape. A mode-switching control program was used to
intelligently select the appropriate frequencies. The process control program allowed the
operator to specify the time interval each frequency would heat the material. The time
49
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intervals could be the same, o r they could vary by simply changing the value before the
next experimental run. Initially, each frequency was given the same time interval value.
0.3 J
0.25 *
« 0.15
0.05 -
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
frequency, f, GHz
Figure 28. Percentage o f Reflected Pow er At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 135 mm And dp = 15.89 mm And Containing
Six-ply Hercules AS4/3501-6 Prepreg
5.3. In p u t Power vs. F requency
For the variable frequency power source, power output varies with frequency
(Figure 29). For example, if the input power was set to 100 W atts, the frequencies with
low power output make the average input power less than 100 Watts. Also, when the
time interval is on the order o f milliseconds, the frequencies are changing very quickly, and
there is a relatively high proportion o f time spent not heating the composite. Higher input
power is needed overall if the composite is to be cured in a short time scale.
50
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1.8
1.6
-
1.4 -
*
0.8
-
"5 0.6 - 0.4 -0.2
-
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
frequency, f, GHz
Figure 29. Incident Power Versus Processing Frequency for Lambda Variable Frequency
Power Source [23]
5.4. Temperature Measurement and Heating Studies Part I: 6-ply Prepreg, Four
Frequencies For V.F. Processing
By placing fluorotropic temperature probes across the Teflon die inside the
microwave cavity, it was found that temperature varied by less than 15 degrees across the
1 Vi" prepreg tape. The tape was only heated to 42°C using 65 to 73 Watts input power
and a time interval o f 100ms for each frequency (Figure 30). The frequencies with low
reflected pow er were judged by measuring their heating capabilities. This was done by
heating a stationary prepreg sample in the cavity and recording data from the temperature
meter. The frequencies that had provided efficient heating were compared for their power
output potential from Figure 27. The frequencies with similar power outputs were chosen
out o f this second set o f characterization studies and were used for additional variable
frequency studies.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
45
40 35 H 30 -
oT
i
middle
outer middle
outer
^ 25 -*
20 10 --
0
100
200
300
400
500
tim e, t, se c
Figure 30. Six-Ply Hercules AS4/3501-6 Heated With Variable Frequency Processing
(2.4285 GHz, 2.444 GHz, 2.594 GHz, and 2.6140 GHz - 100 ms each) At An Average O f
70 Watts And A Pulling Speed o f 0.281 ft/min
5.5. Heating Studies Part H: 18-ply Prepreg, Change In Cavity Dimension
A new set o f experiments was conducted in which an 18-ply piece o f prepreg tape
was processed in the cylindrical brass cavity. It was thought that the thickness o f the sixply tape was not efficiently interacting with the electromagnetic radiation put out by the
power source. The coupling probe depth (dp) was changed, and the cavity length was
again varied. At a cavity length o f 93 mm and a coupling probe depth o f 22.90 mm, it was
found that several modes could be excited. Figure 33 shows there were thirteen
frequencies having below 10% reflected power.
Figures 27, 3 1 ,3 2 and 33 will show the natural selection o f the final set o f cavity
dimensions that was used for many o f the variable frequency experiments. Cavity length,
coupling probe depth, and prepreg thickness affected the frequency sweep charts. By
changing the prepreg thickness from 6-ply to 18-ply, more o f the incident power was
absorbed by the material, resulting in more possible modes that could be excited with
52
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minimal reflected pow er (R.P. < 10%). Increasing the coupling probe depth seven
millimeters decreased the reflected power for the possible modes and the peaks were
tighter, giving a better idea o f the processing frequency needed. Finally, the cavity height
was increased three millimeters. The reflected power was decreased slightly, and there
was a tighter range o f possible modes around the center frequency o f 2.45 GHz, which
was theoretically the m ost efficient processing frequency in the swept range.
0.25
0.2
0)
5o 0.15
CL
■D
O
e0)
&
0.1
0.05
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
frequency, f, GHz
Figure 31. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 90 mm And dp = 15.89 mm And Containing
Eighteen-ply Hercules AS4/3501-6 Prepreg
53
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0.25
0.2
a.
0.15
0.1
0.05
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
frequency, f, GHz
Figure 32. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 90 mm And dp = 22.90 mm And Containing
Eighteen-ply Hercules A S4/350I-6 Prepreg
54
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0.25 -
0.2
i
4)
5o
a
•o
0)
0.1
0.05 r
2 .1
2.2
2.3
2.5
2.4
2.6
2.7
2.8
2.9
-0.05
fre q u e n c y , f, GHz
Figure 33. Percentage o f Reflected Power At 2.00 to 3.00 GHz Processing Frequencies
For A Cylindrical Brass Cavity With Lc = 93 mm And dp = 22.90 mm And Containing
Eighteen-ply Hercules AS4/3501-6 Prepreg
Input power was raised to approximately 100 Watts, and the time interval was set
to 1000ms for each frequency. Each o f the three frequencies heated the composite for
approximately one second before switching to the next one selected. Temperature versus
time profiles (Figure 34) revealed that again the center temperature was higher than the
outside o f the tape. The tape was too thick to exhibit large heating rates with the small
input power used, so the tape thickness was decreased to 11 plies. It was then suggested
that the time interval used to switch among the frequencies not be constant, but that the
time in which the outside heating frequencies are attenuated be increased. This would
allow the outside edges to be heated to a temperature closer to that in the center o f tape,
resulting in a more even distribution o f cure.
55
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60
50 -
O 40 -
I—
3
center
outside
outer-middle
20 -10 -
0
50
100 150 200 250 300 350 400 450
tim e, t, se c
Figure 34. Eighteen-Ply Hercules AS4/3501-6 Heated With Variable Frequency
Processing (2.1173 GHz, 2.4162 GHz, and 2.7014 G H z - 1000ms each) and A Pulling
Speed o f 0.281 ft/min
5.6. Heating Studies Part HI: 11-ply Prepreg, Four Frequencies and Time Intervals
The third set o f experiments involved heating eleven-ply Hercules AS4/3501-6
prepreg tape. Four frequencies (2.1173 GHz, 2.4162 GHz, 2.6328 GHz, and 2.6809
GHz) were used to process the tape at 0.281 ft/min and 50 W atts input power. 2.1173,
2.6328, and 2.6809 GHz were deemed as outside heating frequencies from previous
heating studies. 2.4162 GHz had the highest heating potential out o f the four frequencies;
therefore, the time interval was set to 10ms, while the other three were set at 1000 ms.
Figure 35 shows the heating profiles for this set o f processing conditions. The composite
did not heat at a rapid rate or to high temperature. 2.1173 GHz in TE2n mode was
deemed to be a poor heating frequency by running single frequency heating experiments
on static prepregs. It was replaced by 2.4267 GHz in T E m mode for the remainder o f the
56
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variable frequency heating studies. This change established two center heating frequencies
and two outside heating frequencies. Another modification made was the input power
being raised to approximately 100 Watts. Ideally, the goals were to obtain rapid heating
to a high temperature and an even temperature distribution across the composite prepreg.
50
45
40
35
30
outer-middle
25
center
20
outer-middle
outside
15
10
5
0
0
100
200
300
400
500
600
tim e, t, sec
Figure 35. Eleven Ply Hercules AS4/3501-6 H eated With Variable Frequency Processing
(2.1173 GHz - 1000ms, 2.4162 GHz - 10ms, 2.6328 GHz - 1000 ms, 2.6809 GHz 1000ms) and Using A Pulling Speed O f 0.281 ft/min
Runs were conducted in which the center heating frequencies (2.4162 GHz and
2.4267 GHz) were set at equal time intervals (1000ms and 2000ms) and the outside
heating frequencies (2.6328 GHz and 2.6809 GHz) were set at time intervals o f 5000ms
and 7000ms.
57
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60
50
40
outer-middle
30
middle
outer-middle
20
10
0
0
100
200
300
400
500
time, t, sec
Figure 36. Eleven Ply Hercules AS4/3501-6 Heated With Variable Frequency Processing
(2.4162 GHz - 1000 ms, 2.4281 GHz - 1000 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed O f 0.281 ft/min
By increasing the center frequency time interval, the heating rate o f the tape
increased, but the temperature distribution was not uniform. This can be seen by
comparing Figures 36 and 37. Increasing the outside heating frequency time interval was
detrimental to heating and was discontinued. The tem perature gradient across the width
o f the prepreg increased. Figure 38 shows the results o f for an increase in time interval for
the outside heating frequencies.
58
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100
90
80
U
70
outer-middle
60
i
I
g
middle
50
outer-middle
40
outside
30
20
100
50
150
200
300
250
350
time, t, sec
Figure 37. Eleven Ply Hercules AS4/3501-6 Heated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4281 GHz - 2000 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed O f 0.281 ft/min
90
y
60
-outer-middle
-center
« 40
- outer-middle
H 30
0
100
200
300
400
500
600
time, t, sec
Figure 38. Eleven Ply Hercules AS4/3501-6 Heated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4287 GHz - 2000 ms, 2.6328 GHz - 7000 ms, 2.6809 GHz 7000 ms) and Using A Pulling Speed o f 0.281 ft/min
59
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5-7. Heating Studies Part IV: 11-ply Prepreg, Four Frequencies and Variation In
The Time Interval
The final three experiments involved setting the following time intervals for the
appropriate frequencies: 2000ms for 2.4162 GHz, 1000, 1250, and 1500 ms for 2.4267
GHz, and 5000 ms for 2.6328 and 2.6809 GHz. Since 2.4267 GHz supplied the most
efficient heating, it was paired with the lowest time interval in hopes o f better controlling
the heating rate and temperature distribution o f the composite prepreg. Microwave input
power was approximately 100 Watts for the three trials. Figures 39 and 40 show the two
heating profiles for a change in the time interval for the frequency 2.4267 GHz from 1000
ms to 1500ms.
When the time interval was changed from 1000 ms to 1500 ms, the heating o f the
composite was more rapid - approximately half the time to reach steady state. The
temperature gradient across the prepreg was less pronounced. The increase in the time
interval also allowed the composite to be heated an additional 10°C. The temperature
distribution across the pultruded part varied from a few degrees to 20°C, which is much
improved from the 40°C temperature gradient on the prepreg from center to edge with
single frequency processing at 100 W atts input power.
60
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90
80
70
60
50
outer-middle
center
40
outer-middle
30
20
10
0
0
100
200
300
400
500
time, t, sec
Figure 39. Eleven Ply Hercules AS4/3501-6 Heated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4287 GHz - 1000 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed o f 0.281 ft/min
100
90
u
70
60
outer-middle
O
oo
oc
m
tim e, t, sec
Figure 40. Eleven Ply Hercules AS4/3501-6 Heated With Variable Frequency Processing
(2.4162 GHz - 2000 ms, 2.4287 GHz - 1500 ms, 2.6328 GHz - 5000 ms, 2.6809 GHz 5000 ms) and Using A Pulling Speed o f 0.281 ft/min
61
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5.8. E xtent o f C u re M easurem ent
Extent o f cure was measured for the product from the pultrusion o f an 11-ply
prepreg heated with the variable frequency power source. Temperature versus time
curves are displayed in Figure 37. The processing frequencies and their corresponding
time intervals and input powers used were 2.4162 GHz (2000 ms and 90 W atts), 2.4281
GHz (2000 ms and 87 Watts), 2.6328 GHz (5000 ms and 88 Watts), 2.6809 G H z (5000
ms and 74 Watts). DSC samples were taken from five points across the width o f the
prepreg to investigate the distribution o f cure. The processing speed for the experiment
was 0.281 ft/min. The heat of reaction o f the uncured sample Hercules AS4/3501-6 was
197.5 J/g from DSC. Table 6 lists the results o f the extent o f cure study for variable
frequency processing.
Table 6. Extent o f Cure o f 11-ply Prepreg With Variable Frequency Processing
Position
H eat of R eaction (J/g)
Extent o f C u re (% )
Outside - Left
158.3
19.85
Middle/Outside - Left
142.2
28.0
Center
125.0
36.71
Middle/Outside - Right
130.5
33.92
Outside - Right
153.6
22.23
62
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The graphite/epoxy composite prepreg was successfully heated using variable
frequency processing technology by setting individual time intervals for the processing
frequencies as opposed to having the same time interval for all four frequencies. Pulling
speed was kept constant at 0.281 ft/min for all experimental runs. Higher microwave
input power will allow more rapid heating, heating to a higher temperature, and ultimately,
faster processing speeds.
5.9. Conclusions
Variable frequency processing provided an even temperature distribution across
the prepreg. Thicker samples provided more efficient heating in the variable frequency
study. The cavity was characterized by measuring reflected power versus frequency for
various cavity lengths and coupling probe depths. Frequencies with the highest heating
potential and similar power output were selected to perform curing studies. Time
intervals were increased for the outer edge heating frequencies to allow for even
temperature distribution across the tape. B etter control of microwave heating was
obtained by varying the time interval for the frequency with the highest heating potential.
Variable frequency microwave pultrusion processing o f composite materials looks to be
promising in providing an even cure distribution across the profile.
63
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Chapter 6. Fiber Reinforcement Effects On Microwave Processing Efficiency
6.1. Material Identification and Purpose of Study
The reinforcing o f composites has benefits, which are two-fold. One is that the
fibers absorb the majority o f the load when the composite undergoes some kind o f tensile
or flexural stress. The second benefit is that the thermal conductivity o f the fibers is
typically different than that o f the resin, allowing the energy to couple preferentially into
the fiber or resin. The materials used in the study were a Hercules AS4/3501-6
graphite/epoxy composite and a DA409/E250 glass/epoxy composite. The reason for
performing experiments on both systems is to compare and contrast the ability, or
efficiency, o f microwaves to process each material.
As was discussed before, the dielectric constant is a measure o f the material's
ability to store electrical energy. The dielectric loss factor corresponds to the material's
ability to dissipate electrical energy as heat. The effective dielectric loss factor consists o f
two terms: the loss factor due to the dielectric and a portion that deals with the
conductivity. The materials' dielectric properties play an important role in determining
how well radio frequency heating will work. The dielectric loss factor is a function o f
temperature and extent o f cure. Jow, et al. document that the dielectric loss factor
increased with increasing temperature until the gelation point was reached. The dielectric
loss factor decreased suddenly during the gel and then continued to decrease as the matrix
fully cured. [24] Springer and Lee examined the microwave interaction with both glass
and graphite fiber reinforced epoxy composites. [25] They listed the dielectric properties
o f both material systems, and the graphite/epoxy composite was far superior over the
glass/epoxy - both uncured and cured. [25] I f the graphite/epoxy system has a higher
64
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dielectric constant, it is expected that it will absorb microwave energy more efficiently
than the glass/epoxy system. For the electrical energy the material system does absorb,
the graphite/epoxy system, with a much higher loss factor, is expected to take that
electrical energy and dissipate it as heat more efficiently. Carbon fibers have a thermal
conductivity value an order o f magnitude higher than glass fibers [26], suggesting that
thermal heat transfer will be better than a composite with glass fiber from the inside to the
outside o f the composite part. It is then predicted that the graphite/epoxy system will heat
more rapidly than the glass/epoxy system.
6.2. Experiments
The glass/epoxy system was processed in a single-mode microwave applicator.
Three foot, six-ply DA409/E250 prepreg tape samples were prepared. The cavity was
characterized with the material inserted into the Teflon die to find what electromagnetic
modes could be excited (Table 7), followed by heating studies for each mode (Figure 41).
The most efficient heating mode, namely T E n2, was chosen to further conduct tests on the
6-ply glass/epoxy prepreg tape samples.
Table 7. Heating Modes for the Cavity with Teflon Die and Glass/Epoxy Prepreg
M ode#
1
2
4
5
M ode Type
TM 011
TE O il
TM 111
TE 112
TE311
C avity L ength, Lc, mm
70
101
117
131
152
P ro b e Depth, L p, mm
11
13.94
15.75
9.5
16.33
65
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45 40 -35 TED11
o
30
TM111
i—
Q--. 25 -
TE112
I-
TM311
E
«
20
-
TE311
15 10
--
5 -
0
100
200
300
400
tim e , t, s e c
Figure 41. Temperature versus Time Results for Heating Studies o f Stationary Six-Ply
DA490/250 Prepreg at Approximately 50 Watts
The glass/epoxy prepreg was processed in a T E m mode cavity at 2450 M Hz in the
existing pultrusion apparatus. The tape was not pulled through the cavity to see just how
well the material would heat in the microwave environment. The processing time was
varied from one minute to 15 minutes. For the 7" cavity, the processing time was
assumed to correspond to the residence time o f a differential slice o f prepreg. In the fixed
frequency experiments for graphite/epoxy samples (see Chapter 4), the fixed pulling speed
for variable input power experiments was 0.281 ft/min. When the glass/epoxy was heated
for tw o minutes, the equivalent pulling speed to give the same residence time was 0.292
ft/min.
66
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6.3. Results and Discussion
Figures 42 and 43 show the heating results fo r a six-ply glass/epoxy and graphite/epoxy
prepreg, respectively, at approximately 50 W atts. The centerline temperature at 120
seconds after the pow er was turned on was 31.38°C for the glass/epoxy system and
41,29°C for the graphite/epoxy system. T he extent o f cure was measured on DSC for the
glass/epoxy samples. For the sample heated in T E m mode at 57 Watts for two minutes,
the material had below 10% cure. W ith an idea o f how well the glass/epoxy heated with
microwaves, an attempt was made to pultrude a six-ply sample at 50 Watts and 0.281
ft/min. Figure 44 shows that there was only a 5°C temperature increase over the tape.
35
30
25
middle
20
outer-middle
outside
15
10
5
0
0
50
150
100
200
tim e, t, s e c
Figure 42. Stationary Six-ply Glass/Epoxy Prepreg Heated in T E n2 Mode at 57 W atts for
Two Minutes
67
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60
50 --
40 -
30 •CL
20 410
-
100
150
200
250
300
350
tim e , t, s e c
Figure 43. Centerline Temperature for Six-ply Graphite/Epoxy Prepreg Heated in T E m
Mode at 50 W atts and 0.281 ft/min
outer-middle
middle
100
200
300
400
500
tim e , t, s
Figure 44. Six-ply Glass/Epoxy Prepreg H eat in T E m M ode at 50 Watts and 0.281 ft/min
68
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Pow er was increased to 85 W atts for processing the glass/epoxy samples because
pow er absorption was low. The residence time was also increased in attempt to identify a
point in time where the exotherm o f the polymerization reaction would make the
tem perature rise more rapidly with time. Figure 45 shows the temperature versus time
curves for points across the tape. The temperature did continue to rise for the span o f five
minutes, but the slope o f the temperature-time curve did not increase even w ith the
increase in power. This could be due to poor tuning o f the cavity, which would lead to
more reflected pow er and inefficient coupling with the material. Extent o f cure o f the
above prepreg was measured to be approximately 70%. Figure 46 reveals that for similar
pow er levels, the centerline temperature o f graphite/epoxy is 40°C greater than that o f the
glass/epoxy sample.
50
45 40 -
O
35 30
Sf
2
5
middle
25 T
outer-middle
o . 20 -
E
<D
"
outside
15 10
-
0
100
200
300
400
tim e , t, s e c
Figure 45. Stationary Six-ply Glass/Epoxy Prepreg Heated in TEn2 Mode at 85 W atts for
Five Minutes
69
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90
70 460 50 *
t-
30
10
100
150
200
250
tim e , t, s e c
Figure 46. Centerline Temperature for Six-ply Graphite/Epoxy Prepreg Heated in T E m
Mode at 80 Watts and 0.281 ft/min
6.4. Conclusions
Glass/epoxy and graphite/epoxy prepregs were evaluated as to their
reinforcement's effect on the efficiency o f microwave heating. The graphite/epoxy system
had a higher dielectric constant, suggesting it would absorb microwave energy more
efficiently than the glass/epoxy system. For the electrical energy that it did absorb, the
graphite/epoxy system, with a much higher loss factor, seemed to dissipate the electrical
energy as heat more efficiently with evidence from the temperature-time curves. The
glass/epoxy will heat to similar temperatures as the graphite/epoxy, but it will require a
longer processing time. It is concluded that the graphite fiber / epoxy heats more
efficiently in a microwave environment and heats more rapidly than the glass fiber / epoxy
system.
70
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Chapter 7: On-Line Monitoring
7.1. Possible On-line Monitoring Techniques
Possibilities for an on-line monitoring system are being explored. Possible
variables to be monitored are temperature, dielectric constant, dielectric loss factor, and
extent o f cure. Extent o f cure cannot be monitored directly. It would be deduced based
on the properties o f heat capacity, dielectric constant, and dielectric loss. It would be
possible to correlate these properties with the temperature as long as the chemical reaction
taking place in this process is exothermic. At the completion o f the reaction, the exotherm
subsides, indicating completion o f cure. However, measuring the temperature o f the
material that is moving through the die is difficult. The degree o f cure is related through
the temperature profile o f the material with respect to time and the dielectric loss,
dielectric constant, and heat capacity in the batch study. Then, one can position a probe at
the die exit location where values for the dielectric constant and dielectric loss can be
achieved.
The other on-line monitoring method can be the measurement o f surface
temperature to ensure complete cure. The assumption is that the material would be heated
from the inside to the surface. I f the surface temperature o f the exiting material is
predefined as the criteria to denote complete curing, (which incidentally is possible to
arrive at from the batch curing studies with appropriate assumptions and calculations) one
can use simple probes to monitor the temperature o f the exiting material. By being able to
observe certain characteristics o f the composite during processing, one can get a better
idea on how to control gelation before the material enters the die. Based upon the
71
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outcom e o f the examination o f a possible on-line monitoring system, a strategy will be
developed to implement the desired on-line monitoring and control system into the
pultrusion apparatus.
Previous w ork by Dr. Jinder Jow, et al. [24], included a monitoring method in a
batch microwave process. The brass cavity dimensions were adjusted to allow resonation
o f the 2450 M Hz wave. Initial dielectric measurements were taken for the uncured epoxy
sample. The sample was heated with a specified input power, and microwave transparent
fluorotropic probes were used to monitor the internal tem perature o f the sample. Upon
completion o f the cure cycle, the circuit was reconfigured to include the low-power sweep
oscillator in place o f the 100 W source in order to begin diagnostic measurements using
the sweep frequency method. Temperature, dielectric constant, and dielectric loss were
monitored during convective cooling. Then DSC tests were run to find the extent o f cure.
From the results o f the batch studies, various curves can be generated:
•
Dielectric loss versus temperature at different extents o f cure
•
Dielectric constant versus temperature at different extents o f cure
•
•
Dielectric loss versus extent o f cure
Extent o f cure versus microwave processing time
72
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It is known that both the dielectric constant and the dielectric loss will increase
with increasing temperature. The dipole relaxation time is related to temperature through
an Arrhenius relationship. If temperature increases, the relaxation time decreases and
subsequently increases the dielectric loss factor. Also, the dielectric properties will
decrease with increasing extent o f cure. This is because the dipoles become less mobile as
crosslinking increases. Jow found that at approximately 40% cure, the dielectric
properties became less dependent on temperature and more dependent on extent o f cure.
[24]
If extent o f cure is related through the temperature profile o f the material with
respect to time and the dielectric loss, dielectric constant, and heat capacity in the batch
study, a fluorotropic temperature probe can be positioned at the die exit. It can be initially
assumed that if the surface temperature is approximately equal to the consolidation
temperature, then complete cure has occurred. The monitored temperature can give
values for the dielectric constant and dielectric loss factor by interpolating the curves from
the batch study. DSC will again evaluate the extent o f cure.
7.2. P ultrusion M odel for O n-L ine M onitoring
A pultrusion model is proposed that will relate the necessary variables to establish an on­
line monitoring system. It begins with a two-dimensional energy balance for a rectangular
profile part in Equation 10, [27] where p, Cp, and k are the density, heat capacity, and
thermal conductivity o f the composite, respectively.
73
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H is the rate o f heat generation per unit weight by the polymerization reaction.
Pm’ is the rate o f heat generation per unit volume by the absorbed microwave energy.
( 11)
where Hr is the heat o f reaction = 197.5 J/g from DSC.
The Cartesian coordinate plane for the rectangular profile is such that z is in the
pulling direction and x is across the width o f the prepreg. Note that in Equation 10, Vz is
the pulling speed o f the prepreg through the die. It can be assumed that the heat transfer
in the axial direction is negligible relative to the heat transfer across the prepreg, therefore
making the energy balance one-dimensional ( — = 0). The microwave power absorption
dz
term in the energy balance is dependent on the processing frequency, dielectric loss factor,
and the electric field strength inside the material.
Pm =
7 tf0 S 0S " | E m
2
( 12 )
where f0 = 2.45 x 109 Hz and s0 = 8.854 x 10'12 F/m (permittivity o f free space). The loss
factor is broken up into two terms, one for the loss factor due to the dielectric, and one for
the electrical conductivity.
The magnitude o f the electric field inside the material is a complex calculation, and
the power absorption model by Wei [ 1] gives an accurate account o f the power dissipation
o f microwave energy inside a composite. I f it is assumed that all o f the microwave
74
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incident pow er is absorbed into the composite, then the electric field strength inside the
material will be similar to the electric field strength inside the cavity. The electric field
strength inside the cavity for the transverse electric (TE) and transverse magnetic (TM)
modes can be calculated by taking the cross product o f the vector equations given in
Equations 14-19 for the TE modes and Equations 20-25 for the TM modes below. [19]
The modes in a cylindrical cavity' can be modeled by a computer program in order to
establish the magnitude o f the electric field at a specified cavity height. This can help by
finding the optimum sample height in the cavity and by being able to visualize where the
“hot spots” will be in the cavity as it heats the pultruded part.
For the TE-modes,
(14)
(15)
(16)
E, = 0
(17)
(18)
(19)
75
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For the TM-modes,
Er = —~r~J'i (A^r) cos(/#)sin(&3z)
/c
(2 0 )
(21)
E. = ^ - J l(klr}cos(l&)cos(<k3z)
(22)
/v
H r = - 1 ^ 1 - sin(/ 0) cos(/c3z)
1
(23)
He = - J \ (*,r) co s^ ^ co s^ z)
(24)
H. = 0
(25)
where ki = 2xim/D, k:, = rnt/L, k2 = ki2 + k32, and X = 27t/k.
The documented kinetics [28] for the Hercules AS4/3501-6 composite will serve
as a basis for this section. The gel point for the graphite/epoxy system occurs at an extent
o f cure o f 30% and separates the two prevalent kinetic expressions. When the conversion
is less than 30%, Equation 26 defines the kinetics. Equation 27 gives the kinetics for the
conversion when it is greater than the gel point.
— = (k, + k 2a)(l-<x)(p -a)
ct
(26)
(27)
(28)
(2 9 )
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where R = 8.314 J/mol K
P = 0.47 = ratio o f initial hardener equivalents to epoxide equivalents.
The dielectric properties o f the Hercules AS4/3501-6 composite have been reported in the
literature [25]. The effective complex permitivities o f the uncured material along and
perpendicular to the fiber direction are e* = l-j25000 and s* = 33.0 - j‘53.3, respectively.
The real part o f the complex permittivity equation is the dielectric constant, while the
imaginary part is the loss factor.
The electrical properties also come into play in the energy balance for the material
system. A proposed energy balance to model the microwave processing o f composite
materials [27] can be modified into a pultrusion model. One must take the pulling speed,
Vz, as small incremental steps in the pulling direction, z. Then the chain rule can be
utilized to establish an energy balance equation that is time dependent for temperature and
time dependent for extent o f cure. Originally, the derivative o f temperature with respect
to time was equated with a two-dimensional dependence o f temperature on location in the
composite and a reaction term. Substituting the pulling speed on the left side, the
temperature-time curves are now dependent on the processing speed.
The reaction term is broken down into two terms. The first is the heat o f reaction,
which is multiplied by the derivative o f the extent of cure with respect to time. This is the
heat generated inside the composite by the polymerization reaction. The second term
takes into account the electromagnetic interaction o f the microwaves with the material
system. Pm' is equivalent to the rate o f heat generation per unit volume by the absorbed
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microwave energy. A pow er absorption model must be employed here to accurately
calculate the amount o f heat generated. Pm' is a function o f the processing frequency,
permittivity o f free space, which is a constant, the square o f the electric field strength
inside the material, and the effective dielectric loss factor. A five-parameter power
absorption model was proposed by Wei, et. al in order to simulate the "power dissipation
inside a fiber reinforced composite during microwave processing in a tunable resonant
cavity." [1] The five parameters involved in the model are the power dissipation due to the
electromagnetic (EM) waves from the side and the magnitudes and the polarization angles
with respect to the fiber direction o f the incident TEM waves at the top and bottom o f the
composite.
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7.3. Conclusions on On-Line Monitoring
It is possible to integrate an on-line monitoring system and process control
equipment to a microwave pultrusion line. First, the curves from the batch studies, i.e.,
dielectric data and temperature-time curves, will be obtained. Then, a two-dimensional
energy balance with heat transfer, polymerization reaction, and microwave power
absorption terms will be employed to find the temperature dependence on the profile size,
dielectric properties, and pulling speed. Next, electric and magnetic fields should be
modeled by computer software to calculate where the "hot spots" will be inside the cavity
for the various transverse electromagnetic modes that can be excited. Finally, a previous
microwave power absorption model [1] will be used to define the electric field magnitude
inside the composite. Com puter simulations could then model what would happen in a
microwave pultrusion apparatus with a chosen set o f processing variables. The on-line
monitoring system would allow the operator to easily control the process variables to
optimize the pultruded product characteristics.
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CONCLUSIONS
The microwave pultrusion apparatus was assembled. The 17.78 cm inner
diameter, tunable, cylindrical, batch, brass microwave cavity was characterized by finding
the different electromagnetic modes for different cavity dimensions. Heating studies were
conducted for each o f the modes to find how rapidly each could heat the prepreg and
where the electric field magnitude was large. Fixed-ffequency experiments at 2450 MHz
were run in T E m mode. The microwave input power was varied from 40 to 100 Watts,
and the pulling speed was varied from 0.281 to 1.17 ft/min. Extent o f cure o f the
graphite/epoxy composite was established as a function o f input power and pulling speed
for a microwave pultrusion apparatus. Extent o f cure increased with an increase in
microwave power input and decreased with an increase in pulling speed. At 100 W atts
input power and a pulling speed o f 0.281 ft/min, the center o f the six-ply graphite/epoxy
prepreg was 98% cured.
Tensile strength and modulus were relatively unaffected by the change in process
variables. Flexural strength measurements revealed there to be maxima in strength and
modulus for certain processing conditions. Too high o f power or too slow o f pulling
speed resulted in a brittle matrix. Low input power or fast pulling speeds gave a product
that had a relatively uncured matrix and low interfacial bond strength.
Variable frequency processing provided an even temperature distribution across
the prepreg. The cavity was characterized by measuring reflected power versus frequency
for various cavity lengths and coupling probe depths. Frequencies with the highest heating
potential and similar pow er output were selected to perform curing studies. Thicker
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samples provided more efficient heating in the variable frequency study by absorbing a
greater percentage o f the incident power. Time intervals were increased for the outer
edge heating frequencies to allow for even temperature distribution across the tape.
B etter control o f microwave heating was obtained by varying the time interval for the
frequency with the highest heating potential. Variable frequency microwave pultrusion
processing o f composite materials looks to be promising in providing an even temperature
distribution across the profile.
Glass/epoxy and graphite/epoxy prepregs were compared and contrasted as to
their effect on the efficiency o f microwave heating. The graphite/epoxy system had a
larger dielectric constant, suggesting it would absorb microwave energy more efficiently
than the glass/epoxy system. The graphite/epoxy system, with the larger loss factor,
converted dissipated the electrical energy as heat more efficiently with evidence from the
temperature-time curves. It is concluded that the graphite/epoxy would heat more rapidly
than the glass/epoxy system.
Possibilities for an on-line monitoring system were explored. Possible variables to
be monitored are temperature, dielectric constant, dielectric loss factor, and extent o f cure.
The degree o f cure is related through the temperature profile o f the material with respect
to time and the dielectric loss, dielectric constant, and heat capacity in the batch study.
Then, one can then position a probe at the die exit location where values for the dielectric
constant and dielectric loss can be achieved. A pultrusion energy balance in conjunction
with a five-parameter power absorption model could help provide a basis for an on-line
monitoring system and/or a process control program for microwave pultrusion processing.
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RECOMMENDATIONS
Microwave processing was applied to a continuous manufacturing process for
composite materials. After studying glass fiber/vinyl ester composites, Min Lin suggested
that in future microwave pultrusion research, one should include other resin/fiber systems
so the technology can branch out to new applications. He also recommended improving
the system for high-speed pultrusion by modifying the cavity design, installing a new
pulling mechanism, and integrating a high power source. Finally, he suggested researching
new microwave transparent die materials and combining a part-shaped cavity and
pultrusion die, where the walls o f the die would be the microwave applicator. [29]
Experimental data on the microwave pultrusion o f graphite/epoxy and glass/epoxy
systems was collected. Higher RPM pulling rollers were placed in the line to increase the
maximum pulling speed to 240 cm/s from 3 cm/s. Heating and curing profiles o f the
pultruded material were studied at different processing conditions. To find the optimum
operating conditions for microwave pultrusion processing, the relation between processing
rate, power, and extent o f cure was examined. A variable frequency power source capable
o f 150 W atts input power was used to achieve a more even temperature and cure
distribution across the prepreg.
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It is recommended that the following issues concerning the pultrusion project be
addressed in future work:
• A higher microwave power source is needed to process larger-profiled parts and
to increase processing speeds.
• New mechanical pulling equipment capable o f the higher line speeds.
• N ew die material that has a longer life and is more transparent to microwaves
than both Teflon and ceramic.
• Thermally heated die at die exit o f microwave cavity to see if single-mode
applicator is a viable alternative to a radio frequency preheater.
• Parallel plate dielectric analysis machine at die-exit to assist in on-line
monitoring o f material properties.
• Combine part-shaped cavity ideas with a pultrusion die to make the die walls the
actual microwave applicator.
• Computer program relating all process variables and material properties for
inclusion in a process control program and/or on-line monitoring system.
•M odification o f an existing larger single-mode applicator to operate at 915 MHz
for the continuous processing o f materials. The larger cavity will be useful in the high
pow er experiments.
It is proposed that the existing microwave technology available at Michigan State
University be integrated into a Vermont Pultrusion Test System available from Vermont
Instrument Company, Inc. The laboratory set-up would provide a means by which to
compare and contrast conventional and microwave pultrusion processing, or to study the
two techniques in conjunction. By selecting a resin system, one may study the optimum
83
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cure conditions, microwave input power, die oven temperature, and pulling speeds. A
polyester or vinyl ester resin would be mixed with the appropriate catalysts and additives
in the disposable container. The glass rovings would be led through the pultrusion line to
ensure proper alignment o f the fibers during processing. The rollers would be controlled
electronically and have a maximum pulling speed o f 75 cm/min (2.46 ft/min). The VICO
bench pultruder would produce 3 mm rods. The experiment station would include glass
rovings that pass through a ceramic ring, a resin fiber-wetting system complete with
disposable resin bath containers and roller bars, an aluminum oxide ring which would act
as the preformer, and a VICO preheater, bench pultruder, and postcure oven. A VICO
mechanical torsion tester would be available to measure the product's shear modulus and
its fracture energy in shear.
84
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BIBLIOGRAPHY
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BIBLIOGRAPHY
1. Wei, J. and M.C. Hawley, and J. Asmussen, Jr., "Power Absorption Model For
M icrowave Processing o f Composites in a Tunable Resonant Cavity," ASM/ESD
ACCE. Chicago, (1992).
2. Bruce, R.W., Material Research Society Symposium Proceedings. 124, 3 [Microwave
Processing o f Materials], (1988).
3. Wei, J., Dissertation for the D octor o f Philosophy, Chapter 7, Department o f Chemical
Engineering, Michigan State University, (1992).
4. Wei, J., Shidaker, T., and Hawley, M.C., “Recent Progress in Microwave Processing
o f Polymers and Composites,” TRIP. Vol. 4, No. 1, pp. 18-24, (1996).
5. Hexcel Carbon Fibers, Product Data Summary, Hercules AS4/3501-6, (1998).
6. Adhesive Prepregs for Composites Manufacturers, Product D ata Summary,
DA409/E250, (1998).
7. Munson-McGee, Opportunities for Innovation: Polymer Composites, NIST GCR
90-577-1, pp. 121 (August, 1990).
8. Pat. 4,792,772, J. Asmussen (1988).
9. L. VanKoughnett and J. G. Dunn, J. Microwave Power. 8 (1), 101 (1973).
10. Pat. 5,406,056, M. C. Hawley, J. Wei, V. Adegbite, and Y. Chang (1995).
11. Pat. 2,245,893, A. Cooper and J. M. Methven (1990).
12. Ghaffariyan, S.R. and J. M. Methven, Mat. Res. Soc. Symp. Proc., 189, 135 (1991).
13. Pat. 5,470,423, M. C. Hawley, J. Wei, V. Adegbite, and M. Lin (1995).
14. Meyer, Raymond W., Handbook o f Pultrusion Technology. Chapman and Hall,
N ew York, (1985).
15. Martin Pultrusion Group. Pultrusion Equipment Buyer’s Guide. [Online] Available
http://www.ne-ohio.net/pultrude/equip.htm, M ayl4, 1998.
16. P.K. Mallick, Fiber-Reinforced Composites: Materials. Manufacturing, and Design.
Marcel Dekker, Inc., New York, 1988, pp. 184-185, 201-204, 345-351.
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17. Lin, M. and M.C. Hawley, "Preliminary Tests o f the Application o f Continuous
Microwave Technique to Pultrusion," 38th SAMPE International Symposium. (1993).
18. Shanku, R., Vaughan, J.G., and J.A. Roux, “Dielectric and Thermal Cure
Characterization o f Resins Used in Pultrusion,” 42nd International SAMPE
Symposium, Pre-publication copy, (1997).
19. Montgomery, C.G., Technique o f M icrowave Measurements. McGraw-Hill Book
Company, Inc., (1947).
20. Hull, D. and T.W. Clyne, An Introduction to Composite Materials. Cambridge
University Press, pp. 158-171, (1996).
21. ASTM Standard D 790 - 84a: Standard Test M ethods for Flexural Properties o f
Unreinforced and Reinforced Plastics and Electrical Insulating Materials.
22. Swanson, S.R., Introduction to Design and Analysis with Advanced Composite
M aterials. Prentice Hall, pp. 91-97, (1997).
23. Qiu, Y. and Hawley, M. C., “Computer Controlled Variable Frequency Microwave
Processing o f Graphite/Epoxy In A Single M ode Cavity,” IM PI. 1998.
24. Jow, J., Hawley, M. C., et. al., "On-line M easurement o f Temperature- and Curedependent Dielectric Properties During Single Frequency Microwave Curing and
Comparison with Thermal Curing o f Epoxy Resins", Conference On Emerging
Technologies In Materials. AIChE. 1987.
25. Lee, W.I., and Springer, G.S., "Interaction o f Electromagnetic Radiation with Organic
M atrix Composites," Journal o f Composite Materials. Vol. 18, pp. 357-386 (1984).
26. Han, C.D., and D.S. Lee, “Development o f a Mathematical Model for the Pultrusion
Process,” Polymer Engineering and Science, Vol. 26, No. 6, pp. 393-404, (1986).
27. Wei, J. and M.C. Hawley, "Modeling and Controlling During Microwave and Thermal
Processing o f Composites," MRS Symposium Proceedings. 269, 439. (1992).
28. Lee, W.I., Loos, A.C., and Springer, G.S., Journal o f Composite Materials. Vol 16,
pp. 510, (1982).
29. Lin, M., M aster’s Thesis, Chapter 6, Department o f Chemical Engineering, Michigan
State University, (1995).
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Phone: 716/482-0300
Fax: 716/288-5989
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